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I am attempting to email an address on my own box (username@domain.tld), and postfix is attempting to deliver to 1@domain.tld - the username is replaced with the number 1. This message is included in mailer-daemon's response:
<1@domain.tld> (expanded from <user@domain.tld>): host mail.domain.tld[private/dovecot-lmtp] said: 550 5.1.1 <1@domain.tld> User doesn't exist: 1@domain.tld (in reply to RCPT TO command)
Searching the mail log, the first result that refers to 1@domain.tld is this:
Oct 19 13:41:44 servername postfix/lmtp[16232]: A547EC29C2: to=<1@domain.tld>, orig_to=<username@domain.tld>, relay=mail.domain.tld[private/dovecot-lmtp], delay=0.19, delays=0.16/0.01/0.01/0.02, dsn=5.1.1, status=bounced (host mail.domain.tld[private/dovecot-lmtp] said: 550 5.1.1 <1@domain.tld> User doesn't exist: 1@domain.tld (in reply to RCPT TO command))
username@domain.tld exists, and can send mail outwards. 1@domain.tld does not exist, obviously.
So why is Postfix repeatedly delivering all mail to 1@domain.tld?