Some users can't suddenly send out

[r2tech]

Hi,
We are having an issue all of a sudden with some users.
They used to be able to send out from our server fine, but in the last few days they suddenly can't.
They are not blocked on the firewall (can ping and access other server resources).
Tried using port 25 and 26 but didn't work. Simply times out.
Tried turning on outbound authentication but didn't work
Same account works fine from a PC in a different network.
Incoming is working fine.
Sending out via ISP SMTP works fine, so no issue with the PC.

This has happened to different domains / user accounts.

Can anyone suggest anything else I can try / check?

Thanks

[thewebhosting]

Kindly check the email logs for the exact error message. You will get the logs from /var/log/exim_mainlog.