[exim] TLS error on connection

One of our business partners is trying to send us automated emails from one
one their system, but is failing.

Normal emails from person to person between our companies are working fine.
However, every time they try to send a system generated email I get the
following error in my logs and they get the email bounced back

2015-05-25 14:02:35 TLS error on connection from <remote_mail_server>
[999.9.999.999] (SSL_accept): timed out

Can anyone suggest anything I can do to fix the problem at my end, or do we
think the problem is that their end?

I am seeing the same error message from a small number of other servers too,
but they mainly seem to be SPAM servers.

Gary