Hi,
I have the same error. But I don't see any limits reached in Plesk (just 6% usage of space (hardquota), mailboxsize 120M from unlimited).
But sometimes I get:
<date> <server> /usr/lib64/plesk-9.0/psa-pc-remote[6109]: handlers_stderr: DATA REPLY:452:4.2.2 Mailbox full#012DEFER
<date> <server>...