On 19/06/2020 10:18, Nick Tait wrote: > Hi David. > > I think I can guess what your problem is, because I had exactly the > same symptom with a different bulk email provider... > > Basically this sounds like an MTU issue: The SMTP client > (mailomta12-sa.btinternet.com[213.120.69.18] in your case) is able to > establish the TCP connection to your server, and the first few SMTP > requests and responses work fine, because the packets are small. Then > when it tries to transmit the content of an email (after the DATA > command), the packets get blocked because they exceed the maximum size > that will fit through your Internet connection, and this causes the > TCP connection to stall, and inevitably time out.
If you have administrative control of your broadband router, then you should increase the MTU of its ISP-facing Ethernet interface by 8 bytes. Of course this only works if it's capable of jumbo frames but most are these days. In my case that's eth0, and this is the interface via which I set up the PPP connection to the ISP: eth0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1508 inet 192.168.2.2 netmask 255.255.255.0 broadcast 192.168.2.255 ether 00:08:a2:0a:89:95 txqueuelen 2 (Ethernet) RX packets 5654036 bytes 5241700471 (4.8 GiB) RX errors 0 dropped 9031 overruns 0 frame 0 TX packets 4027451 bytes 1179885101 (1.0 GiB) TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0 device memory 0xdfda0000-dfdbffff As you can see I've set its MTU to 1508 bytes such that the PPPoE overhead drops it back down to 1500, matching the rest of my network. The PPP configuration also sets the MTU to 1500 for good measure.
