Fixed sinceThe problem described in this KB article has been completely fixed since Parallels Plesk Panel 10.4.4 MU#27 (see release notes).
Postfix is refusing some messages with the following error:
milter-reject: END-OF-MESSAGE from mx01b.domain.tld[188.8.131.52]: 5.7.1 Command rejected
CauseWhen Postfix receives mail for a mailbox that is over quota, the psa-pc-remote milter will reject the message, causing a "5.7.1 Command rejected" response to the DATA command. The message is never queued, and the sender never receives a "Mailbox full" response.
WorkaroundAs a temporary workaround, you can try restarting the pc-remote service:
# /etc/init.d/pc-remote stop
Stopping psa-pc-remote: [ OK ]
# /etc/init.d/pc-remote start
Starting psa-pc-remote: [ OK ]