Most instances of this error stem from misalignments between the two main Postfix configuration files: main.cf and master.cf .
Think of it as a letter carrier arriving at a house, seeing the lights on and hearing a dog bark, but finding no doorbell and no address number. They can’t deliver the letter, but they can’t send it back as undeliverable either. So they stand on the sidewalk, holding the envelope, muttering, "Try again later." Most instances of this error stem from misalignments
The receiving server is up, but it refused to accept the message at the TCP or TLS level. Causes include: So they stand on the sidewalk, holding the
If you use for virtual domains, users, or aliases, a broken connection is the #1 culprit. postfix/upd acts as a proxy between the main Postfix process and your database. often sit between Postfix and the final destination
often sit between Postfix and the final destination. If these services crash or if the connection port (often 10024 or 10026) is blocked, the transport becomes unavailable. Chroot & Permission Issues
To (add v flag and increase verbosity):
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