SMTP error "bad string length 0 < 1: myhostname ="

In my server everything seems to work OK, but SMTP-service is claimed to be down (Monitor / Status of Services / "SMTP-Server: Offline") and also /var/log/mail.log shows that something is wrong (after 24.4. 13:00:08 UTC).

Code:

...
Apr 24 12:58:53 server1 pop3d: Disconnected, ip=[::ffff:xxx.yyy.zzz.xyz]
Apr 24 13:00:01 server1 pop3d: Connection, ip=[::ffff:127.0.0.1]
Apr 24 13:00:01 server1 pop3d: Disconnected, ip=[::ffff:127.0.0.1]
Apr 24 13:00:01 server1 imapd: Connection, ip=[::ffff:127.0.0.1]
Apr 24 13:00:01 server1 imapd: Disconnected, ip=[::ffff:127.0.0.1], time=0
Apr 24 13:00:08 server1 postfix/sendmail[25714]: fatal: bad string length 0 < 1: myhostname =
Apr 24 13:05:01 server1 pop3d: Connection, ip=[::ffff:127.0.0.1]
Apr 24 13:05:01 server1 pop3d: Disconnected, ip=[::ffff:127.0.0.1]
Apr 24 13:05:01 server1 imapd: Connection, ip=[::ffff:127.0.0.1]
Apr 24 13:05:01 server1 imapd: Disconnected, ip=[::ffff:127.0.0.1], time=0
Apr 24 13:05:01 server1 postfix/sendmail[25818]: fatal: bad string length 0 < 1: myhostname =
.... 

Same continues after restart

I did not made any changes (even access to server on that day) so cannot understand this change....

Which file I should modify?

System info

Thanks again for your support, everything else works lovely....

 

Is myhostname set in /etc/postfix/main.cf?

 

Thanks for hint - above was empty (i.e. myhostname= )

When adding "myhostname=www.example.com" (or similar ) and restart
- all monitoring green again
- alarms not coming any more to Monitor / "Mail-Warn - Log"
- sending with N900 / TSL-login to SMTP working now like a dream.
(also normal OK (login only)
- message even delivered

I just love this, when both IMAP and SMTP with TSL...

Thanks, sun is shining again to my server