In this scenerio, it was related to an accidental "chown" on "/etc/init.d" causing all permissions to be messed up. For postfix the following can be run to identify possible issues:
/etc/init.d/postfix check postfix/postfix-script: warning: not owned by group postdrop: /usr/sbin/postqueue postfix/postfix-script: warning: not owned by group postdrop: /usr/sbin/postdrop postfix/postfix-script: warning: not set-gid or not owner+group+world executable: /usr/sbin/postqueue postfix/postfix-script: warning: not set-gid or not owner+group+world executable: /usr/sbin/postdrop
So by running the following you can solve the discovered issues:
chown -R root:postdrop /usr/sbin/postqueue chown -R root:postdrop /usr/sbin/postdrop chmod g+s /usr/sbin/postqueue /usr/sbin/postdrop killall -9 postdrop postfix reload
After running you can check the following files for errors:
nano /var/log/mail.info nano /var/log/mail.err
Further to this in Virtualmin I ran:
Which returned a value for the domain, but it showed and odd error in:
So, this could be corrected with:
chmod 4755 /usr/bin/procmail-wrapper
Then sit back and watch some e-mails... all ok it seems!
NB: I did a further issue (possibly network related) stating that a Perl module (URIDNSBL.pm line 701) had an "uninitialized value" - no obvious solution even with Google's help.
UPDATE: What I didn't spot/mention was an error just prior to this one stating "Received.pm line 357". The solution was found and implemented and only required installing 3 perl modules by running:
apt-get install libnet-dns-perl