Hi Brujo,
Having another look at your latest log - entries, I come to the conclusion, that the "error" - message might be misleading.
Your log shows, that the eMail is coming from an EXTERNAL eMail - account, is delivered to an INTERNAL ( hosted domain on your server ) eMail account, while you setup the recipient eMail address to forward to an EXTERNAL eMail - address.
Now let's look, how the whole process has been done:
Message "10385A08C3" comes in, "limit out" - handler is being skipped, quota-handler is being skipped, external SPF is being checked, passed and put to queue.
Postfix passes over to DKIM - check, dk_check verifies with a "PASS" and message goes back to queue.
Plesk-sendmail handler takes over as the next handler, but is not responsible, but still logs with
Code:
Oct 20 20:56:20 mail journal: plesk sendmail[12898]: handlers_stderr: PASS
Oct 20 20:56:20 mail journal: plesk sendmail[12898]: PASS during call 'limit-out' handler
Oct 20 20:56:20 mail check-quota[12900]: cannot get sender domain
Oct 20 20:56:20 mail check-quota[12900]: Unable to intialize check-quota mail handler
Oct 20 20:56:20 mail journal: plesk sendmail[12898]: Error during 'check-quota' handler
... which is actually not relevant, as the handler should be skipped.
"postfix/pickup" is next to pick up the ( temporary delivered eMail ( #10385A08C3 ) ... only temporary, as there is a FORWARDER for the eMail - account! ) message ( now with a new message ID #5A2FAA08C5 ! ), which will now forward the message to the EXTERNAL eMail - account and reports the action with a successful deliverey in your logs.
... while at the same time, "postfix/pipe" logs the delivery action over plesk_virtual service as being completed.
Conclusion: No error, just an irritating failure message from "
plesk.sendmail", which isn't involved, but is still a valid handler in your message process.