A user has pointed out that the absence of an error when sending is not a reliable or satisfying confirmation that a message has been sent. This is particularly the case when the sending progress bar might flash up only briefly, if at all, and be lost in the spell-check dialogue ("Check spelling before sending"). The user could check one or more Sent folders or request a DSN, but these are less immediate forms of feedback. A similar request from 2006 is here: <http://forums.mozillazine.org/viewtopic.php?f=39&t=469099>. There might be said to be a deficit in the feedback given to cautious users. I can see this requirement as being satisfied in one or both of two ways: 1) A preference under "composition" (maybe even on by default) whereby the "Sending messages" bar remains on the screen until cleared. Gmail for example leaves a status box on the screen until the next mouse click. 2) A status bar update to confirm "Message sent" with the name of the account, and the first-mentioned recipient. This would also be copied to the Activity Manager, which at the moment only really seems friendly to power users, and doesn't apparently include sending. Partly this demand has been met through the status message cache of Mail Tweaks add-on, or the "Progress History" extension, but either were updated for TB 3.1. Note that the Progress History docked nicely with the status bar, which the Activity Manager does not. There are also suggestions <http://forums.mozillazine.org/viewtopic.php?f=29&t=2234081> that the activities in the activity manager include progress or status, a comparison with Eudora's status window <http://forums.mozillazine.org/viewtopic.php?f=28&t=1910165> and more comments on the benefits of Progress History here <https://getsatisfaction.com/mozilla_messaging/topics/thunderbird_should_display_a_status_window_while_receiving_sending_mails>. See also Bug 404814.
I think Outlook also does not notify any better.