First thing I'd try is running a Windows PC in Windows safe mode, to test the effect of startup apps like AV, backup software, VPN etc., which are bypassed in safe mode.

Thanks to sfhowes for a very helpful suggestion but no real difference in behaviour in safe mode.

We started with 3 people complaining about SMTP error messages and bing unable to save messages to the Sent folder - this has now gone up to 6 people.

No changes to any of the settings.

The only assumption I can make is that however Thunderbird makes its SMTP connection it is not stable with the office network environment and internet connection we have.

I have never used Thunderbird before and whilst some clients are functioning properly, having many that are unstable is not good.

I have checked telnet connections to port 465 and 993 from the affected machines - all good - so no obvious reason why Thunderbird should be unable to connect. The internet latency on a ping to the server is a bit high, typically around 60ms, up and download speeds are 16Mbs and 3Mbs so e-mail should be no trouble.

The problem machines do have multiple e-mail accounts set up, all on the same mail server. (I have all 20 accounts set-up on a Thunderbird Client at home and no issues - 6ms ping 250MBs down 70Mbs up)

Any ideas on how to troubleshoot or adjust settings to make the connection more tolerant would be appreciated.

If all accounts work fine on your home network, but some don't work on the office network, with the same server and TB settings, I'm inclined to think the issue lies with the office network hardware. If you were to set up an account on an unstable PC with a different mail client, it might indicate whether the issues are TB-related or independent of the client, i.e. related to the network or possibly the PC hardware. An intermittent problem is probably not caused by the static settings of a mail client.

We tried Outlook and it seems to work fine so the issue at least in part appears to be with Thunderbird.

Based on some comments in another forum we deleted all the accounts from some of the office machines and re-created these from scratch. This has allowed emails to be sent without errors although now the sent mails are not being saved to the sent folder in the Inbox.

This message pops up: Your message was sent but a copy was not placed in your sent folder (Sent) due to network or file access errors. You can retry or save the message locally to Local Folders/Sent

We tried adjusting security.tls.version.min from the default of 3 down to 1 and the save to sent folder has improved from always failing to intermittent fail.

A check of the site SSL shows it only supports TLS 1.2 and 1.3. Simulated device handshakes nearly all show connections on TLS 1.2. Thunderbird docs suggest that security.tls.version.min set to 3 uses TLS 1.2 so changing it to 1 should not make any difference but it has.

Very confusing!

Whatever is happening, the off site Thunderbird machine always works and so far Outlook on site has not created any issues, so Thunderbird seems to be sensitive to something in the network environment.

Any thoughts on settings to improve Thunderbird connectivity? Or possible network issues to look out for?

The error about saving to Sent sometimes occurs with providers like MS or gmail accounts (or accounts based on those systems) that automatically copy sent messages to Sent or Sent Mail (gmail), so 'Place a copy in' in the Copies & Folders section of Account Settings should be unchecked, and the correct target for sent mail should be subscribed.

I can't offer much advice about network troubleshooting, but if the problem is confined to just some PCs, I would look at the integrity of the network adapters. If they use wifi, it's fairly easy to test with a different USB wifi adapter. The existing adapters might only support TLS 1.1.