Hello All,
I am delighted to report that I have found and fixed my problem.
I first followed Viktor's advice and increased the debug level for email
from BT.
This did not reveal much more than we already knew; my server was timing
out waiting for data.
I then followed up the MTU issue suggested by Nick and Jan.
I found that the MTU for my router was 1492, but the MTU for my network
card was 1500.
I followed these instructions
https://support.purevpn.com/how-to-change-mtu-value-on-linux to find the
best MTU value.
My thinking was that this might find additional problems at my ISP or
beyond.
Perhaps not surprisingly the best MTU value turned out to be 1492, so I
set my network card to 1492.
Since then I have received two emails from btinternet.com.
Once again thank you all for your help; I would not have found a
solution without it.
With best wishes.
David
On 19/06/2020 15:14, Jan Ceuleers wrote:
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.
