Ok, found an important clue in this subject (I think!)
Although ISPconfig tells me that smtp server is online (see first post) I cannot telnet port 25.
It times out on the external host
It times out on localhost (when i first SSH to the server and from there telnet to port 25)
Combined with the fact the log file of the mailserver is empty this is probably the cause of the problem.
Questions that remain:
- why is ISPconfig telling me the SMTP server is online
- how to proceed with the error?