Tricky to guess at. Turn off more features, I guess. You already tried chunking. Next would be fastopen, then pipelining.
However, given it was right after all the data (even in non-chunking) one has to wonder if it's a content-check of theirs going wrong. Does a given failing message get through on a later retry, or fail for ever? I guess another thing to try would be to build your own Exim from source, in case there's some fix Deb haven't picked up. Unlikely, though. -- Cheers, Jeremy -- ## List details at https://lists.exim.org/mailman/listinfo/exim-users ## Exim details at http://www.exim.org/ ## Please use the Wiki with this list - http://wiki.exim.org/
