[Exim] SMTP not responding

I'm not sure if this is an Exim problem, but if would possibly like some
help please.
I have various users who when they try and send mail, they receive an error
message that the SMTP server has not responded in 60 secs.......
The strange thing is that some users are fine.
I am quite happy to read up on the problem, if I knew where to look.

Thanks