[exim] How to debug SMTP connection lost while reading messa…

Hi,

I have a mail server in a company network that tries to connect my exim smart
host. The company has a very slow internet connection and needs to send some
quite big messages (10-20 MB). Transferring one of these messages should take
about 15-20 minutes by their upload speed.

However these big messages rarely get delivered. In Exim's log I see many
messages like this one:
"SMTP connection from <host> (<internal name>) [<ip>] lost while reading
message data"

On the Exim side I don't do any special blocking and the messages do not hit
size restrictions for sure, smaller messages get through without problems.

I don't think there is some kind of MTU problem (my servers are in
'reasonable' data centers and I don't block ICMP)
I tried sending a big message from the company server to another email
provider and that seemed to work without problems.
I tried using another Exim smarthost located in a different data center but
same the same problems as described above.
I used tcpdump on my Exim server and it looked like the complete message was
being transferred but before Exim acknowledged successful message reception,
the connection was dropped (I could not detect in wireshark what did cause
this - any hints welcome).

Therefore I guess it's some kind of timeout on my Exim's side.

Any hints what to try next? Can I configure Exim timeouts that may influence
it's behavior?

fs