Mozilla Thunderbird is one of the most powerful email clients. However I used to find that it used to keep the active window open while sending emails. This was a bit irritating specially if the email has large attachments.
On searching for background send, I found that the feature exists but doesn’t show up in the preference. You need to enable it.
After enabling it, it works very well. The only thing you need to be careful is not to send an email and immediately close your email client or shut down your PC. What happens is that email will not go as it take a few minutes (depending on the size of the email) for the email to go. The good part is that the email don’t disappear, they still in the Outbox. It will try to re-send the next time you start Thunderbird, but there is no way for you to know if its gone or not, unless you check.Read more