Have you checked DNS resolutions are correct through the VPN ? Found an issue with DNS resolutions failing on latest Thunderbird & VPN over the weekend Try enabling DNS over HTTPS, step by step instructions in the topic below if you need them.

https://support.mozilla.org/en-US/questions/1454425

Thank you very much!

Unfortunately, this did not solve my problem. However, I left one thing out of the recipe, so that may be influencing my results.

I checked the network address for my relay mail server for outgoing mail. It appears to be correct. I did this while running vpn. I did not disconnect from vpn and try the nslookup again.

I followed the instructions in your post above for turning on DNS over HTTPS in thunderbird. (I am on a Mac, so my thunderbird menus are slightly different. I had to get to the settings page by the "Thunderbird" menu which gave me a "Settings" menu item. But after that the navigation was the same.)

I restarted thunderbird. It logged me in without requesting my password. I do not know whether this is from my employer's single-sign-on or from the saved password in thunderbird.

I tried to send one message. It succeeded, as always.

Then I tried to send a second message. It failed, as described in my original post.

What I did NOT do was remove the saved password from thunderbird before trying this, as you had done in your instructions. So that may have thrown off my results.

I am just chicken. You see, I have a workaround for this problem in that thunderbird will let me save the draft message and then send it from an outlook client. But see my related post from yesterday, for which I do not have a workaround:

https://support.mozilla.org/en-US/questions/1454411

If I disconnect vpn and remove my saved password from thunderbird, I might be able to re-login to thunderbird and I might not, and sometimes that problem lasts for weeks, and during that time I have to read all my mail using the outlook client, which is a huge productivity killer, which is why I use thunderbird in the first place :)

So, next time I experience the problem 1454411 from above (which will probably be within a few weeks), and I am unable to login to thunderbird anyway, I will follow the rest of your instructions.

Meanwhile, I've left thunderbird configured to use DNS over HTTPS.

Thank you so much.

Thank you very much! Unfortunately, this did not solve my problem. However, I left one thing out of the recipe, so that may be influencing my results. I checked the network address for my relay mail server for outgoing mail. It appears to be correct. I did this while running vpn. I did not disconnect from vpn and try the nslookup again. I followed the instructions in your post above for turning on DNS over HTTPS in thunderbird. (I am on a Mac, so my thunderbird menus are slightly different. I had to get to the settings page by the "Thunderbird" menu which gave me a "Settings" menu item. But after that the navigation was the same.) I restarted thunderbird. It logged me in without requesting my password. I do not know whether this is from my employer's single-sign-on or from the saved password in thunderbird. I tried to send one message. It succeeded, as always. Then I tried to send a second message. It failed, as described in my original post. What I did NOT do was remove the saved password from thunderbird before trying this, as you had done in your instructions. So that may have thrown off my results. I am just chicken. You see, I have a workaround for this problem in that thunderbird will let me save the draft message and then send it from an outlook client. But see my related post from yesterday, for which I do not have a workaround: https://support.mozilla.org/en-US/questions/1454411 If I disconnect vpn and remove my saved password from thunderbird, I might be able to re-login to thunderbird and I might not, and sometimes that problem lasts for weeks, and during that time I have to read all my mail using the outlook client, which is a huge productivity killer, which is why I use thunderbird in the first place :) So, next time I experience the problem 1454411 from above (which will probably be within a few weeks), and I am unable to login to thunderbird anyway, I will follow the rest of your instructions. Meanwhile, I've left thunderbird configured to use DNS over HTTPS. Thank you so much.

No probs. To be honest this sounds like a cache issue. When you first start Tbird, it pulls all the connection info correctly as a new session, then on the second attempt it has cached some of the info so reuses it. It could be because the 'new email' has downloaded in between (especially if that sent by you email has been received) and as you mentioned the passwords for the SMTP server and IMAP are different. You could check if this is the issue by turning off checking for new mail on start up, time period etc and just have manual check on. Then restart Tbird. Send one email, then try a second without downloading email. If that works then try a download of new email and try a third send, and see if that works.

'Thunderbird does not have this problem when I am connected directly to my site's internal network, not using vpn.'

Just a thought, when you have the VPN running, do you also have an active local connection as well ? My husband WFH and when he used to have the wi-fi interface running as well as the hardwired connection/interface running a VPN it caused no end of issues. WAN IP addresses different as one your IP provider and one the company WAN/VPN provider/software. Plus DNS issues.

If you do, you could try turning off the local interface. When I run a VPN (on windows10) my local connection shows as 'No internet access' but the VPN 'Internet access'. Might also want to flush the DNS cache to ensure you get a valid test.

Intermittent problems are the worst, can be a complete nightmare to solve! This would also explain the office connection difference. No competing systems. Good Luck!

>Just a thought, when you have the VPN running, do you also have an active local connection as well ?

Good idea to check, but no, I just have the one network interface, wireless.

Okay, this may be solved! I really can't believe it after living with it for 8 months.

I usually update my thunderbird version only when my employer's IT department says to. They have to picky about new versions of things and it is best to wait till they've approved an update.

Today I was told to upgrade to thunderbird 128.0. (I was on 115.12.2.) What the thunderbird website gave me was 128.0.1esr. And the 'send' problem reported in this post seems to be FIXED.

Right after I upgraded and restarted thunderbird, I sent a message. It succeeded. No surprise.

But then I sent another message 2 minutes later, which normally would have failed. But it also succeeded. Messages sent at intervals over the next 2 hours all succeeded.

So was this release 128 or was it the "DNS over HTTPS" setting just now taking effect now that I had the new release?

I turned off "DNS over HTTPS" and restarted thunderbird. Tried to send a message, it prompted for my password, and sent it.

At intervals over the next 11 minutes sent additional messages. It did not prompt for my password and continued to send the messages.

While it is sending, it says "sending message, connecting, sending login info, checking mail server capabilities"...all of which goes by so fast I can't catch every word.

So, did version 128 solve this problem?

It looks like it!

Thank you, jennydeione for all the suggestions!

And thank you, Thunderbird, for release 128!

>Just a thought, when you have the VPN running, do you also have an active local connection as well ? Good idea to check, but no, I just have the one network interface, wireless.

So do you use any LAN stuff from the laptop ? printer, music speaker etc Also more ideas to check do you just have your ISP's router or your own ? If you can look at the config settings for the (your/home) router can you make sure that the WAN-NAT settings are enabled correctly for your VPN type.

That's brilliant !!! Glad something has sorted it for you. Since you are on a lucky streak, do you know anything about electrical engineering, by chance :-) ? Have an issue that I have been battling 12 years now !!!

>So do you use any LAN stuff from the laptop ? printer, music speaker etc Also more ideas to check do you just have your ISP's router or your own ? If you can look at the config settings for the (your/home) router can you make sure that the WAN-NAT settings are enabled correctly for your VPN type.

No other lan stuff from laptop.

Thanks, all good things to check.

> Glad something has sorted it for you. Since you are on a lucky streak, do you know anything about electrical engineering, by chance :-) ? Have an issue that I have been battling 12 years now !!!

Ha! Sorry, no EE knowledge. Now, you could ask me about chocolate chip cookies (where to get the good ones, not how to make them), and I might show a little more expertise ;)