Mailer-daemon and "Mail Delivery Subsystem" emails in Gmail
An email from "Mail Delivery Subsystem" looks alarming, but it is just Gmail's automatic notice that a message could not be delivered.
"Mail Delivery Subsystem", from mailer-daemon@googlemail.com, is Gmail's automatic bounce message: an email you sent could not be delivered. Open it and read the error code and reason, then fix the address or the problem and send again. If you get these for emails you never sent, someone has probably forged your address; check your Sent folder and secure your account.
What "mailer-daemon" means
A daemon is a program that runs in the background. The mailer-daemon is the part of a mail server that reports delivery problems. In Gmail it appears as Mail Delivery Subsystem and usually starts with "Address not found", "Message not delivered" or "Delivery Status Notification (Failure)". Other providers use names such as "Mail Delivery System" or "Postmaster". It is a normal email bounce-back.
Common reasons and fixes
| What it says | Typical code | What to do |
|---|---|---|
| Address not found / user unknown | 550 5.1.1 | Check the spelling, or ask the person for their current address. |
| Domain not found | No MX record | The part after the @ is misspelled or the domain no longer receives mail. |
| Mailbox full | 452 or 552 | Try again later or contact them another way. |
| Message too large | 552 5.3.4 | Send a link instead of the attachment. See attachment size limits. |
| Blocked or rejected as spam | 550 5.7.x | The receiving server refused it. Remove links or attachments, or check your domain's SPF, DKIM and DMARC. |
| Delivery delayed | 4xx | A temporary problem. Gmail keeps retrying; no action is needed yet. |
Look up any code in the SMTP error code library, and see soft vs hard bounces.
Bounces for emails you never sent
If your inbox fills with Mail Delivery Subsystem messages about emails you don't recognize:
- Check your Sent folder. If the messages are not there, a spammer most likely put your address in the "From" line of their own mail. The bounces come back to you. This is called backscatter, and your account is usually fine.
- If the messages are in your Sent folder, someone is using your account. Change your password right away and turn on 2-Step Verification.
- At the bottom right of Gmail on a computer, select Details next to "Last account activity" to see where your account is signed in, and sign out of anything you don't recognize.
- Run Google's Security Checkup at
myaccount.google.com/security-checkup.
If you own a domain, publishing a DMARC policy makes it much harder for others to forge it. See the deliverability checklist.
Watch for fake bounce messages
Scammers copy the look of bounce messages: "Your messages could not be delivered, click here to release them". A real Gmail bounce never asks you to sign in, pay or download anything. If in doubt, read how to tell if an email is legit.
Getting lots of bounces from a list?
If you send to a list and many messages bounce, your list has old or mistyped addresses. Too many bounces can hurt your sender reputation. Clean the list before the next send.
Questions people ask
- Is mailer-daemon a virus?
- No. Mailer-daemon is the automatic system that reports failed deliveries. But be careful with messages that only look like bounces and ask you to click a link or open an attachment.
- How do I stop mailer-daemon emails?
- Fix the address you are sending to. If they are backscatter from emails you never sent, they usually stop on their own; mark obvious junk as spam rather than filtering out all bounces, so you still see real ones.
- Why did my email bounce if the address is correct?
- The receiving server may have refused it as spam, the mailbox may be full, or the message may be too large. The reason and code are in the bounce message.
- What is mailer-daemon@googlemail.com?
- The address Gmail uses to send bounce notices. googlemail.com is an older Google mail domain, still used for these system messages.