I assume the "1st account" is defined as the "default"

Try selecting a folder on the left for one of the other account and clicking write, does it use the correct SMTP (the one defines in Tools menu (alt+T) > Account settings) or the default.

If you have an email tab open, is the account chosen the account for the mail in the tab. The choices of "default" changes based on what is selected. That is why there have been multiple add-ons written over the years to force the issue.

This is the problem. Selecting a folder on the left, I see the SMTP settings changed. And I did not change it. Before upgrading to TH24.5, I used TH10 for quite a long time, and I never had this issue. Now I decided to upgrade, and this particular issue popped up. I never know which smtp is carrying out my messages. In some cases the destination recognises that the smtp does not match the sender domain and throuws the message into the spam folder. I never know when my correspondents receive my messages. It is really annoying.

Thunderbird has not changed the way it handles SMTP servers since like version 1. There has been no change at all between V10 and V24.

The SMTP changes based on what account your in on the folders on the left, y6ou have an SMTP specified for each account and is changes to whatever is appropriate. Apparently you thin it should remain static for all accounts. No correct behaviour, but there are add-ons that enforce that, but I told you that 5 days ago. here is a link https://addons.mozilla.org/en-US/thunderbird/search/?q=smtp&appver=24.0&platform=windows the first two look relevant

Hello Mat.

thanks for your reply.

I already use SmtpControl. Actually, now I traced the problem precisely to this add-on.

After reading you msg, I remembered that this was the only thing that changed. Before TH24, I used "SendVia". Everything worked like a charm. Each account would send email through its own smtp.

When I upgraded, "SendVia" stopped working (it works up to TH10, but it's no longer supported) so I had to look for something else.

I found "SmtpSwitch", but that one also stops working with TH13, So I installed SmtpControl. This add-on, for some reason keeps the last smtp used and "shovels" it into the account settings, actually changin it. I have been struggling to configure it in the "right" way (if there is one) but manual does not exist and I got no results.

So I just uninstalled SmtpControl. And I am happily back in having each account sending via its own smtp server.

I lost the ability to dynamically switch the smtp at the "send" instant, but I really use it seldomly. I hope someone will write a new plug-in foe that purpose that works.

Thank you for putting me in the right track to find the problem.