[exim] Final 250 OK not being received

I have an odd problem. Servers from online-age.net which are runing
sendmail are not receiving my final 250 OK message. So they keep the
connection open waiting and finally time out. My end think the message
is received and processes it. The sending end thinks the message failed
and resends it resulting in duplications.

I'm beginning to think that this is happening with more than one host
and may be the cause of some random email duplications. Is there any
chance Exim is closing the connection too quickly at the and of a
sucessful SMTP session and the sender isn't getting the final OK?