SMTP Problem - 451 local error processing

[ctek]

i have the following strange problem
users are not able to sent email over plesk/qmail smtp service with authentication. there is an error

451 requested action aborted, local error in processing

i checked then with telnet from different computers and it seems, that connections from official registered IP addresses with reverse dns are accepted to be connected, but computers with IP address from dynamic subnet are rejected with the above error.
so, my users are therefore not able to send email over this smtp.

any clue what this could be ?
is there some rbl / reverse dns check im not aware of?
where to check this ?

 

[Hedgren]

Pls check, if the MailRoot/message.id file is deleted or corrupted. The problem could be in the attempt to open a message.id file somewhere, presumably to write the messageID of the message to it.
(and just in case: also check permissions and the structure of /var/qmail/ directory).