postconf -n | grep transport_maps
Postfix will try to connect, fail, and generate the "unknown mail transport error."
The error occurs globally for all emails, and checking systemctl status opendkim or systemctl status amavisd shows the service is stopped or deadlocked.
openssl s_client -connect smtp.sendgrid.net:587 -starttls smtp -crlf postconf -n | grep transport_maps Postfix will try
After a yum update , the error appears only for certain transports. SELinux audit logs ( /var/log/audit/audit.log ) show AVC denied messages.
If you are running Postfix in a Docker container, postqueue -f might be necessary after every configuration change to clear the error.
By default, many Postfix services run in a chroot jail (a restricted environment) for security. Services like smtp or smtpd are often configured in /etc/postfix/master.cf to run chrooted. If you are running Postfix in a Docker
# Check Amavis (Common Content Filter) sudo systemctl status amavis # Check Dovecot (Common Mail Delivery Agent) sudo systemctl status dovecot # Check ClamAV / SpamAssassin sudo systemctl status clamav-daemon sudo systemctl status spamassassin Use code with caution.
Check for missing fields. Every service row must have exactly 8 columns (Service, Type, Private, Unpriv, Chroot, Wakeup, Maxproc, Command).
If the transport name is perfectly correct in both files, the error can be thrown because Postfix is physically blocked from executing the transport binary. Chroot Restrictions # Check Amavis (Common Content Filter) sudo systemctl
Generate a custom policy module if needed (using audit2allow ).
Many default Postfix installations (especially on Debian/Ubuntu) run daemons inside a "chroot" jail for security. If a system update upgrades a core library (like libc or libresolv ) but the files inside the chroot jail are not updated, the transport agent will crash instantly.
If the partition containing /var/spool/postfix is full, or if the destination mailbox has a filesystem quota issue, the transport agent might crash or return an unexpected exit code. Check disk space with df -h . 4. Clearing the "Suspended" Status
a cryptic signal that your mail server has encountered a structural mismatch between its instructions and its available tools