Mail Delivery Subsystem: Delivery Failure 101

Mail Delivery Subsystem: Delivery Failure 101

Globally, there are over 4 billion active email accounts or addresses, sending over 300 billion messages daily. 

Email is the most popular online communication tool, more popular than social media or instant messaging apps. Yet, many people using an email account today still have no idea how it works.

The system responsible for delivering a message from one email user to another is called a Mail Delivery Subsystem. It may also be called the mailer daemon, message transfer agent, mail delivery system, or mail transfer agent. 

It is responsible for transmitting electronic mail between addresses, or from one computer (the sender) to another (the receiver) using one of the three main protocols: SMPT, IMAP, or POP.

This article will dive deep into a mail delivery subsystem and the protocols it uses to send messages, including errors that you can encounter with this system, how to fix the errors, how spammers take advantage of it to trick users, and more.

Basically, this is your comprehensive guide to the mail delivery subsystem.

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What is a mail delivery subsystem?

It is the software system that sends a message from one email server to another and then, finally, to the intended recipient.

Email messages are exchanged between mail servers, which are physical servers hosted across the globe by email providers. When you send a message from your email address, it goes from your email server to that of the intended recipient. The mail delivery subsystem is responsible for routing this message between the servers.

If the mail delivery subsystem experiences an error while delivering the message, it will report back to you. That's why you may occasionally get messages from a 'mailer daemon' or 'Mail Delivery Subsystem' with a subject line similar to "Failed Delivery". 

These messages are automatically generated and include the date and reason for the failed mail delivery so that you can correct it if possible. Millions of people experience the same problem every day, but few people know the cause.

sending email

Why does the mail delivery subsystem send you messages?

This system is responsible for giving the email users the delivery status notification. If, for whatever reason, your message is not sent, it has to report back and alert you to the issue. 

Otherwise, you'll assume that your message has reached the intended recipient's inbox and you'll end up waiting for a reply letter that never comes.

Each email provider has a unique address for their mail delivery subsystem. For example, if you have a Gmail account (whether it's "@gmail" account address or a custom domain), is the address that will notify and give you feedback about failed email message deliveries.

As mentioned, millions of people experience this same problem every day but few of them understand the cause.

If you want to know how to stop mail delivery subsystem messages, you'll need to get to the root of the problem.

These are the primary causes of email delivery failures:

1. The recipient's inbox is full

If the recipient's account mailbox has run out of allotted cloud storage space, there will be no space in which to store the message. That means the receiving email server will bounce back the message to the sending server. 

You'll then receive a delivery status notification message telling you that the recipient's mailbox is full and your message could not be delivered.

2. Message blocked

Your message can be blocked by the recipient's mail server for many reasons. The recipient may have defined several rules and settings for their mail server to block a message. 

For example, if it is sent to too many people simultaneously or if the message contains a suspicious URL link. 

Such rules are set to prevent spam messages, but you can get caught up in the spam folder net even if you're sending an innocent original message.

3. Address not found

This is the most common reason for a mail delivery subsystem not to deliver a message. 

If you typed an invalid address, then the message has nowhere to go. You may have made a mistake in the recipient's address, for example, >, so if you see this error, that's the first thing to check. 

If you correct the spelling error and still receive no response to the same message, then the account likely doesn't exist.

address not found

4. Error codes

Your message can also get blocked due to errors in the receiving email server. You will get a message from the mail delivery subsystem containing the specific error code. 

Common error codes in mail delivery system emails include:

  • 500: The receiving server could not process the commands from the sending email server.

  • 501: Syntax error detected in commands from the sending email server.

  • 502: Commands not implemented.

  • 503: Bad sequence of commands.

  • 541: The message was rejected by the recipient's address.

  • 551: The recipient's mailbox does not exist on the receiving server.

  • 552: The mailbox does not have enough storage.

  • 553: Mailbox name does not exist.

  • 554: Message failed without additional details.

How do I address mail delivery subsystem errors?

1. What to do if the recipient's mailbox is full

If you receive an error message indicating that the recipient's email account is full, then you will have to find another way to contact them about the problem. 

Tell the recipient to increase their cloud storage so that they can start receiving important electronic mail. They can also provide an address to forward your messages to while they sort out their storage issues, so ask them for the address or tell them to check their forwarding settings.

2. What to do if it says "address not found"

If the message from the mail delivery subsystem says the address is not found, then you likely made a mistake when typing it. 

The first step is to double-check the address and domain on your original message for any mistakes, then correct them. Afterwards, resend your updated original message. 

If you receive the same error message again, then it's likely that the account or domain does not exist.

To improve your email deliverability and stop your emails from going to junk  (instead of your recipients’ inboxes), explore our latest posts.

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3. What to do if it says "message blocked"

If the mail delivery subsystem indicates that the message was blocked before reaching the recipient's inbox, then you'll need to find a way to contact and verify the intended receiver, telling them about the problem. 

The person can then figure out how to tweak their defined email rules so that your message will get delivered.

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Malicious activities tied to a mail delivery subsystem

1. Phishing

Spammers and scammers never run out of techniques to exploit email accounts for monetary gain. 

One major technique involves impersonating a mail delivery subsystem message to trick you into clicking a suspicious link or opening a file attachment. If you click the link or open the file, you can introduce malware to your PC or mobile phone, or you may end up sharing sensitive details unknowingly. 

This is known as phishing and you should always report phishing to your email provider. Phishing is the most common form of cybercrime and causes billions of dollars in losses each year. Google alone blocks over 100 million phishing emails daily, and other popular email providers fight similar levels of phishing activity. 

It is mandatory in this era to know how to recognize phishing attacks and avoid becoming a victim.

To avoid falling victim to phishing attacks, never click any link or open any attachments from a message claiming to come from a mail delivery subsystem. An original message from this kind of system won't contain a link to click or files to open. 

Likewise, always check the address from where the mail bounce message comes. Every email provider has its unique mailer daemon that you should be able to find with a quick Google search. 

If the supposed email delivery status notification comes from an address other than the original one listed on Google, then it's likely fake.

Also, endeavor to log and report phishing emails to your email provider. They can take action to block the address from sending any more messages to addresses on their server, and prevent the hacker from tricking any more people with a suspicious link or file, thanks to your effort.

2. Spamming

Another technique that malicious actors use is to hack into legit email addresses and use them to propagate spam mail messages. 

This can happen if you get tricked into giving up your email account's password by a phisher, or if a hacker gets hold of your password via a data breach. 

In this case, you'll likely receive multiple notifications of failed email deliveries. This happens because spammers send messages and letters to many addresses (Over 80 billion spam messages are sent every day).

If you suddenly start receiving a lot of mailer daemon notifications for messages that you did not send, then your email account may have been hacked or compromised. 

If this occurs, change your password immediately to a strong one that you've never used before, and install antivirus software to scan your device for malware and remove any threats it detects.

Also, send an original message to your contacts, utilizing the "broadcast list" feature, warning them that your email has been compromised. Tell your contacts to be wary of any unusual message, possibly with a link or file attached, that comes from your account. 

Lastly, you can report the issue to your email provider to see if they can help.

Steps to avoid malicious activities tied to the mail delivery subsystem

1. Always check the sender's email address

Hover your mouse over the "from" address to confirm that it is legitimate. Your email provider has a unique mailer daemon address, so double-check to ensure that it is actually what sent the message. If not, then it's likely a scam. 

You should also watch out for slight spelling errors that hackers use to trick users, e.g, to impersonate (1 instead of i).

2. Be wary of attachments

Most legitimate platforms don't send attachments for users to open. Instead, they redirect you to a place on their website where you can view documents. If a supposed email delivery status notification comes with an attachment, it's more than enough reason to be suspicious. 

Avoid opening any such attachments. If you really want to, then ensure that it is first scanned for malware or viruses before opening it.

email delivery failure

One of the tricks that spammers and hackers employ is to place a well-known URL (e.g, in their message, but clicking on the link takes you to another malicious link that exposes you to the risk of getting hacked. 

Always use your mouse to hover over any URL and ensure it redirects to the link that is actually displayed. Avoid clicking links entirely if you notice a suspicious sign anywhere in the message.

4. Check for generic greetings

Electronic mails from spammers tend to begin with generic greetings such as "Dear user", "Dear friend", "Dear valued customer", "Dear [your email address]", etc. Most legitimate emails will call you by name instead of using these generic greetings. 

So, be wary of any purported mail delivery status notifications that contain similarly generic greetings.

Explore other articles from our blog to learn all about transactional email types and best practices.

Common email transmission protocols

Your email messages are delivered thanks to certain protocols that direct the delivery of such a form of email from the sender to the intended recipient inbox. 

The most common email delivery protocols include:

1. Simple Mail Transfer Protocol (SMTP)

An SMTP server receives the message from the email client and then passes it on to another SMTP server to relay the message to the intended recipient. 

A mail user agent (MUA) initiates the handshake with the SMTP server and relays the message to the mail transfer agent (MTA), which checks the IP address of the intended recipient and routes the message to the correct server.

SMTP servers are not inherently secure; they don't have in-built encryption or security mechanisms. So, to protect emails from being intercepted, email providers add extra security features to the server such as Secure Sockets Layer (SSL) encryption.

SMTP is the most common email transfer protocol. It's used by most email service providers including Mailer To Go, which notably lets users connect third-party applications to the SMTP server endpoint for extra functionality.

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2. Post Office Protocol (POP)

This protocol involves executing download and delete operations on the email messages. 

When a POP client connects to an email server, it retrieves all the messages from the mailbox and stores them on your local device. 

That means that you can keep a copy of your messages and access them offline. After downloading and forwarding the messages to your device, they get deleted from the remote server.

POP is designed to receive emails but not to send them. That's why you need SMTP to handle the sending part.

3. Internet Message Access Protocol (IMAP)

The Internet Access Messaging Protocol (IMAP) allows you to access your messages directly on the email server. You can manipulate the protocol and its settings to organize your emails into folders and subfolders, permanently delete some messages, set or remove email flags, and more. 

All messages remain on the server until the user deletes them. This Internet standard supports multiple simultaneous connections to an email server.

These are the three main protocols that email service providers employ to ensure that your messages get to the intended recipient's inbox and that electronic mails sent to your address actually arrive instead of getting blocked or bounced. 

You can learn more about these protocols by watching the following video.

When your email client experiences a problem with one of these protocols, you'll receive a message from the mail delivery subsystem notifying you of the error. When you get such a message, refer to the sections above where we listed the most common types of errors and what you can do to solve them.

Explore Mailer To Go’s informative blog to improve your email deliverability, discover the difference between transactional and bulk emails, prevent subscription bombing, and more!


You've likely received multiple messages from a mailer daemon or mail delivery subsystem in the past, as it's a frustrating but fairly common scenario. 

If so, this article was published specially for you. We have explained what the mail delivery subsystem is and the common reasons why you might receive messages from it. 

We've mentioned various errors and how you can rectify them and also gave you a better idea of how hackers attempt to use the mail delivery subsystem to scam unsuspecting users, along with the steps to take to avoid falling victim.

At this point, you should have a clear idea of how the mail delivery subsystem works, so you'll be less confused—and hopefully less frustrated—the next time you come in contact with it.

By employing a secure service like Mailer To Go, while understanding the protocols and security tips outlined in this post, your chances of being compromised by mailer delivery subsystem risks will be considerably lower.

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Frequently asked questions

What is a mail delivery subsystem?

A mail delivery subsystem, also known as a mailer daemon, is responsible for delivering your emails. It's a program that works in the background to ensure your emails reach their intended recipients.

If an email can't be delivered for some reason, the mail delivery subsystem sends a notification to the sender.

Why am I receiving mail delivery subsystem messages?

If you're receiving messages from the mail delivery subsystem, it usually means that an email you tried to send couldn't be delivered. This could be due to a number of reasons, such as the recipient's email address being incorrect or their inbox being full.

The mail delivery subsystem message should provide more information about why the email wasn't delivered.

How can I fix mail delivery subsystem issues?

Fixing mail delivery subsystem issues often involves identifying why the email wasn't delivered and addressing that issue. For example, if the recipient's email address was incorrect, you would need to correct it and resend the email.

If you're using a transactional email service like Mailer To Go, they may provide additional tools and support to help you troubleshoot and resolve delivery issues.

Can mail delivery subsystem messages be spam?

While most mail delivery subsystem messages are legitimate notifications about delivery issues, it's possible for spammers to forge these messages in an attempt to trick you. Always be cautious of unexpected messages and never click on suspicious links.

If you're unsure about a message, contact your email service provider for assistance.

What does a "Failed Delivery" message from the mail delivery subsystem mean?

A "Failed Delivery" message from the mail delivery subsystem means that an email you tried to send couldn't be delivered.

The message should include more information about why the delivery failed, such as the recipient's email address being incorrect or their inbox being full.

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