I started getting a “couldn’t save sent message” error message in thunderbird.
I logged onto the server and restarted courier-imapd and postfix and retried. Still the same error.
I checked the maillog and found this error.
Mar 12 17:33:53 plesk postfix/postfix-script: warning: not set-gid or not owner+group+world executable: /usr/sbin/postdrop
As usual, a quick run of the command
Confirmed it was a problem,
So I ran the fix permissions command.
Apparently this is not a well known command so I have posted here.