In my experience, sending email in Thunderbird takes an average of 5 seconds or more with just a popup "sending" sitting there. Why not put it in background processes like the other email clients.
I've never seen this implemented in a way that isnt confusing. It goes to some "Outbox" and you no longer understand why it stuck there and how often it's retrying. You can also have weird failure modes where the mail is sent but not saved to the Sent folder. This is also not obvious to display in a background process
I prefer email sitting in the outbox for a short period of time before send/receive completes, it has saved me from mistakes enough times that it is worth the delay.
But.. it seems like a solveable problem..