> Tomasz K. Jarzynka: > > Finally, I ran a tcpdump on our origin mail server, our firewall > > and the destinantion mail server (thanks to the help of its > > administrator) but the output is inconclusive to me. On our side, > > It looks like transmission stalls after a couple hundred bytes + > > subsequent retransmissions eventually leading to a timeout. On > > the destination side, it looks like the connection is ended > > immediately after the start of transmission. > > Google for > 1 - IP path MTU discovery. > 2 - TCP window scaling.
This doesn't look like a PMTU discovery problem, more like a NAT screwup. A duplicate ACK packet #78 in mnisw-fw.dump seems to get translated into a packet #81 in mnisw-mailhub.dump with a wrong sequence number and with no NAT translation. This causes your mailer to RST the seemingly new session, this RST is then seen by the remote side as an original session reset. Mark
