[exim] Connection timed out errors

Can anyone suggest a tips how to diagnose suddenly getting some
percentage of connections to other servers "timed out" but the network
is ok

1) tail exim main log for something like:
H=mail.example.net [1.2.3.4]:25 Connection timed out

2) manual telnet to 1.2.3.4 port 25 and see connection is ok

How to find why exim thinks it is timing out? Is there a internal set of
flags exim keeping? If yes, how can I reset? Thank you for the help