Sean Durkin: [ Charset windows-1252 converted... ] > Hi Viktor, > > Am 10.09.2014 um 16:19 schrieb Viktor Dukhovni: > > Have you tried disabling TCP window scaling? It might be confusing > > some middle-box (firewall, NAT device, ...) on path between the > > remote systems and your MTA. > I would not have thought of that... I've tried that now, but it does not seem > to help. > > > Post the hostname/IP address of the receving system. > mail.tuxroot.de > > > Capture and examine a tcpdump recording of mail from one of the > > problem senders. Any sign of retransmission by the sender? > I'm trying to get a good dump and will post results once I get one. > Not that easy since the external hosts keep changing all the time. All mail > affected comes from mass mailers that use server clusters, so I keep getting > those messages from lots of different remote hosts. I'm waiting for it to > happen from one of the hosts I've seen before. > > Retransmission is tried numerous times, but for every retransmission > the lost connection message is the same (identical number of bytes), > as far as I can tell. That's one thing that puzzles me... So e.g. > a message is delivered twice and each time the connection is lost > after exactly 17441 bytes, even if it's different remote hosts > trying, that's kind of odd.
No, it means the same problem is happening. Same error, same symptom. > What I do notice there is that in fact the connection seems to be > *very* slow. In the above example, the whole process takes several > minutes. I don't have any throughput or network speed issues with > other hosts, though. I've tried sending mail from Gmail, Yahoo, Slow performance is typical for TCP window scaling problems. Have you tried to turn it off in your kernel? # sysctl -w net.ipv4.tcp_window_scaling=0 To make it permanent: # echo 'net.ipv4.tcp_window_scaling = 0' >> /etc/sysctl.conf Wietse