[Exim] Can't connect to exim from client

Hi there,

I'm rather new to exim and linux so sorry if this is a silly question.

I have a remote user who is having trouble connecting his outlook client to
pick up his mail (pop3).

The logs simply state:
------
SMTP command timeout on connection from [remote-user-machine-ip-address-etc]
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The mail server is behind a firewall with a tunnel opened from the remote
users ip (simple port forwarding).

Any ideas what the problem is?

TIA,
Dave