have you tried without an anti virus sitting in the middle of the transaction? Disable email scanning in the anti virus product and see if that helps. The scanning significantly slows mail downloads in almost all circumstances and at time is the system is busy or the anti virus particularly bad it can cause the connection to actually time out.

Email scanning does not add any real security that does not already exist in the anti virus product. Hence Microsoft's decision to not have dedicated mail scanning in defender.

Hi Matt

I am running AVAST with the mail shield not installed. To double check I just tried again with AVAST completely disabled - no difference to the problem.

So I don't think it is the antivirus.

Thanks anyway, a good thought.

Some further information.

Below is my status bar before the silent fail (all looking good).

Plus an example of the end of the POP3 log - it just stops part way through a line. Not always the same line, can also be part way through the data after the RECV:

Try the elimination process;

• Restart Thunderbird with add-ons disabled (Thunderbird Safe Mode). On the Help menu, click on "Restart with Add-ons Disabled". If Thunderbird works like normal, there is an Add-on or Theme interfering with normal operations. You will need to re-enable add-ons one at a time until you locate the offender.
• Restart the operating system in safe mode with Networking. This loads only the very basics needed to start your computer while enabling an Internet connection. Click on your operating system for instructions on how to start in safe mode: Windows 10, Windows 8, Windows 7, Windows Vista, Windows XP, OSX

If safe mode for the operating system fixes the issue, there's other software in your computer that's causing problems. Possibilities include but not limited to: AV scanning, virus/malware, background downloads such as program updates.

Finally restart Thunderbird in safe mode with the operating system is in safe mode, just to be sure.

It may help to increase the mailnews.tcptimeout preference in Tools/Options/Advanced/General/Config. editor from its default, 100 secs., to 300 secs. It is mentioned in this article about sending large attachments, but much of it applies to downloading on slow connections.

Thanks Matt and sfhowes.

I have tried Thunderbird in safe mode, no improvement.

I have not been able to try it in windows safe mode - I have my email stored on an encrypted drive that will not mount in safe mode, would take some work to change for testing, might test over the weekend.

My mailnews.tcptimeout is already at 1200.

The large email that was clogging my inbox finally downloaded last night on the slow connection, after 12+ hours of attempts every 10 minutes.

I can easily download 100mb files using download managers or firefox on my slow connection, but can't download 5mb emails. Have had this problem for over 1 year.

I wonder, is with your slow connect an ancient router? This is something from the dusty archive of version 3 and should not really be relevant. But it might be.

https://thunderbirdtweaks.blogspot.com/2010/10/i-cant-sent-mail-networktcpsendbuffer.html

Thanks Matt

I have now tried this one (network.tcpsendbuffer) without improvement. I am accessing the internet via a mobile phone hotspot on the slow connection (no router involved).

Made me think about TCP settings generally, I tried TCP Optimizer - did not fix the issue.

I have had the issue now using Thunderbird with two completely different ISP (and thus different POP3 servers).

Would it help to log something other than the POP3 logging in Thunderbird? The fact that no POP3 errors are being logged might suggest something - just cuts out mid-download?