Plesk and qmail: 421 temporary envelope failure (#4.3.0)

I stumbled upon a server running Plesk 8.2.1 where a certain user could not receive e-mail. I sent an e-mail to the user from my mail client, and I never saw it enter the user’s mailbox. It didn’t even appear in the logs.

After checking the usual suspects, like MX records, mail account configuration, and firewalls, I was unable to find out why it was occurring. Even after a run of mchk, the emails would not be delivered.

I began testing with a telnet connection to the SMTP port:

$ telnet 11.22.33.44 25
Trying 11.22.33.44...
Connected to 11.22.33.44.
Escape character is '^]'.
220 www.yourserver.com ESMTP
HELO domain.com
250 www.yourserver.com
MAIL FROM: test@test.com
250 ok
RCPT TO: someuser@somedomain.com
421 temporary envelope failure (#4.3.0)
QUIT
221 www.yourserver.com
Connection closed by foreign host.

Temporary envelope failure? I was still confused. After reviewing the logs, I found the following line whenever I tried to telnet to port 25 and send an e-mail:

Dec  2 00:15:49 www relaylock: /var/qmail/bin/relaylock: mail from 44.33.22.11:17249 (yourdesktop.com)

It turns out that the customer was using greylisting in qmail with qmail-envelope-scanner. After a quick check of /tmp/greylist_dbg.txt, I found the entries from me (as well as a lot of other senders), and that ended up being the root of the problem.