Since Thunderbird is contacting another computer at the time of sending, there is a period beyond which the cancel is no longer an option because the connection is in progress. I presume you have not set the proper protocol and all is well (?)

Thanks.

It's correct that protocol was wrong, but that is not the point. - I did a mistake and changed to correct port, instead of changing protocol which would change default port to the right one.

You can always kill threads either if they are in progress. So removing the dialog you as user think this is over, but it's not.

So really think they should clean up this nicely as dialog is aborted.

I've been running Thunderbird for more than 10 years and until just a few years ago the calls to empty email and download(I run POP3) as you started TB, not checking if there were an internet connection up or not.

So this also had some anomalies to it, every account to empty timed out just doing startup. - then they fixed up this, checking if internet is up before call to empty accounts.

So thinking in the same way about this SMTP calls - kill threads properly - so this does not take 5-10 years too

If you wish to file a bug, you can do so at bugzilla.mozilla.org. Please be as explicit as possible, so that a developer can readily reproduce it. I respect what you're saying, but telling me doesn't get it filed, and I don't have your expertise on this to define it properly. Thank you for your interest.