mailer-demon 

An evil supernatural force that lives within cyberspace and randomly rejects sent emails for no apparent reason.

Dave: Did you email that hottie from Craigslist about hooking up this weekend?

Ryan: I tried but the mailer-demon apparently is already dating her...because he sent it right back to me.

mailer-demon by Not John Vallone November 6, 2010

mailer-daemon 

A cyber-fiend who hijacks your e-mail transmissions for horrific ends and instantly declares your message a failure.

"...that I may have your hand in matrimony." SEND

MAILER-DAEMON: < message failure>

" O scowl of night!"

mailer-daemon by cisco djidd June 28, 2009

[e-mail] 

The mailing person who sends your emails back to you with a failure for no visible reason.

aaand, send. Not again! mailer-daemon sent it back once again!
[e-mail]: Message failed to send to <[e-mail]>

[e-mail] by Griggles October 19, 2020