We all have to deal with them at times: the “mailer daemon” (or “mail administrator” or “mail delivery subsystem”) messages that show up in our inbox to let us know an email message we sent did not make it to its destination. Sometimes the reason is obvious—we spot a typo in the email address we used for the recipient—but often we can’t see what the message is trying to tell us. I hope this helps demystify things a bit.
Some mailer daemon messages may show up immediately, and some may take a day or so to appear. They are automated responses that are generated by computers, not handled by humans (i.e., it’s nothing personal). The contents of mailer daemon messages will (sometimes cryptically) spell out the reasons for the error, and there will generally be an error number listed.
- 550 Invalid recipient — there’s something wrong with the way the address is typed (the part that comes before the @ symbol), or the person closed that account and doesn’t have that email address any more
- 550 host not found — there’s something wrong with the “domain” part of the address (the part after the @ symbol)
- 550 JunkMail rejected — either the recipient’s mail provider, or the sender’s provider, is blocking the message as spam
- 550 “username” Is Not Accepting Mail From This Sender — your address is on someone’s “Blocked Senders” list
- 552 message too large — mail systems will often block large messages (message size limit may be anywhere from 5 to 25 MB)
- 552 Mailbox full — recipient’s mail storage has reached capacity (no incoming messages will be accepted until messages already there get deleted)
If the message spells out the first thing listed (invalid recipient), it might be a good time to check your Contacts list (aka Address Book) for people who need updating or deleting. First check/fix/delete the one(s) specified in the mailer daemon message. Then, while you’re in your Contacts, look for other entries that seem unwanted or obsolete.
[Side note: It’s a good idea to make a backup copy of your Contacts info once in a while.]
A flood of messages may be a sign your email account has been hacked
If you suddenly start seeing a lot of mailer daemon messages (and you know you can’t possibly have been sending out that many messages to problematic destinations), the problem might be more sinister.
- It’s possible that your email account has been hacked/compromised, and someone it using it to send out spam. If you think that’s the case, change your password immediately, to something you’ve never used anywhere before (and don’t use that password for anything else). Also, if your account has been hacked, check other mail settings to make sure your mail hasn’t been set to be forwarded to another address. [Upcoming blog posts on dealing with email account hacking and creating good passwords; see a site like this to check the strength of your passwords, or a site like this for more info on the topic of password strength.]
- Make sure your security software is up to date, and run a full system scan. In addition to your regular security, it might be a good idea to download and run the free version of MBAM (Malwarebyte’s AntiMalware) as a sort of second opinion, to make sure your computer is as clean as your regular security program is reporting.
Step 2 above pertains to Windows users, as Mac users don’t have as much to worry about with computer infections (at this point). But regardless of the kind of computers people have, users of Gmail, Yahoo mail, Hotmail, and AOL mail are more likely to be targets of email hacking, since those are account types with a broad user base, and where all of a person’s data (messages and contacts) are stored on a server somewhere.
I hope this helps take some of the mystery out of those messages. If you have questions, please feel free to put them in the comments, and I’ll be happy to answer them.