Quote:
Process did not exit cleanly, returned 255 with signal 0
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Postfix seems to be calling a different process to deliver the mail, and this process failed with an error.
Postfix then requeues the mail to try again later, assuming that mail delivery failed.
My suggestion: try to find out what program is called by postfix to deliver the mail.
The transport configuration (main.cf and transport.cf files) should help you with this.