Postfix service unavailable error

451 4.7.1 Service unavailable - try again later after connecting is indicative of a problem with the queue - most likely postfix cannot write the queue file, and hence is forced to temp reject your message.

As Dennis said, check the availability and free space on your /var/ partition - this holds both the postfix queue and the log file.

Another possibility is that smtpd_milters = inet:localhost:12768 is not running - but that would not explain the absence of logs.

Just check /var/log/syslog

In my case it was problem with security:

Jan 11 17:57:34 konkib opendkim[2375]: mail._domainkey.mydomain.com: key data is not secure: /etc/postfix/dkim/mail.private is in group 110 which has multiple users (e.g., "remotebkp")
Jan 11 17:57:34 konkib opendkim[2375]: 24B2112081D: error loading key 'mail._domainkey.mydomain.com' Jan 11 17:57:34 konkib postfix/cleanup[17011]: 24B2112081D: milter-reject: END-OF-MESSAGE from localhost[127.0.0.1]: 4.7.1 Service unavailable - try again later; from=<[email protected]> to=<[email protected]>

It works after editing /etc/groups.