Thunderbird gets an acknowledgement from the SMTP server that it has received the message without error before it takes down the sending mail dialog. If it receives an error then your notified immediately in that send dialog. If there is no error from the SMTP server Thunderbird's role in sending mail is done and dusted. There is a whole other delivery status notifications process if the delivery fails after that point. The fact that about 1 in 1000 email servers on the internet have it turned on is not something anyone but the server administrator can do anything about. There is a huge difference between sent and delivered, and I think your looking at delivery and trying to align it with sending. The RFCs for email make no guarantee of delivery. Nor do they guarantee notification of delivery or failure of delivery. (A lot like the postal service really. You pay your money. You give them the letter and they will try and deliver it) Easy resending of mail from the send folder is as simple as right clicking and selecting edit as new or right clicking and selecting Forward as > attachment and adding a new mail suggesting they may not have received your earlier correspondence. Note here that any interference in sending by third party programs such as anti virus and security software almost guarantees that email will not work as advertised, as security software vendors appear to be of the understanding that standards do not apply to them.