Try these settings: outgoing server name: smtp.office365.com on port: 587, security: STARTTLS, authentication: normal password, User name: full email address.

http://kb.mozillazine.org/Hotmail

Thankyou! This solved the problem though I still don't understand why my original smtp address doesn't work (and suddenly stopped working). Does anyone else have the same problem?

sorry it didnt work ... it is doing it on a hotmail account and its random ... sometimes it will and the next it wont and its happing on more than 1 email

could it have something to do with using it alot as the last while we have been sending and receving more emails then usual

also it looks like its sending out 2 emails for 1 reply

I've also noticed that (when I'm "live" - not using Thunderbird) Hotmail is slow to render and "overlays" folders sometimes when I move mailboxes. I'm not a conspiracy theorist but I remember vaguely when broken links were (deliberately) put out by Internaff Exploiter or something else Windows.Could this be another attempt to make me go all cloudy, upgrade to "Premium" and follow The One True Path? Mind you, I also remember Lynx (a TEXT-based browser), downloading Linux onto 3.5"floppies via narrowband, 8mg hdds and children playing outside so maybe technology's just leaving me behind? I also downloaded and ran CLAMAV but no problems. I'm running Mint 18.2 "Sonya" and whatever version of Thunderbird comes with it and update at least daily and this workaround was perfect. Could it be software you're using? I still lean towards Microsoft fault...

integra22 said

sorry it didnt work ... it is doing it on a hotmail account and its random ... sometimes it will and the next it wont and its happing on more than 1 email could it have something to do with using it alot as the last while we have been sending and receving more emails then usual also it looks like its sending out 2 emails for 1 reply

Check that you don't have a security program scanning outgoing mail, as these can often interfere, especially with secure connections. Most mail providers have limits on the number of messages sent or received per hour, day etc., but it's usually a matter of it being in the hundreds.

Hotmail/outlook automatically copies sent messages to the Sent folder, so you should uncheck 'Place a copy in...' under Copies & Folders in Account Settings.

ok ill check that im using free avg right now and ill uncheck the box for copies thanks

Outlook Support have just told me it's DEFINITELY a Thunderbird problem. So any ideas why changing smtp server solved the problem?

integra22 said

ok ill check that im using free avg right now and ill uncheck the box for copies thanks

There was another user who had similar problems until they removed Avast (AVG is owned by Avast); start your computer in Windows safe mode to test if AVG is a factor.

flyingspaghettimonster said

Outlook Support have just told me it's DEFINITELY a Thunderbird problem. So any ideas why changing smtp server solved the problem?

I don't know exactly why changing the server solves the problem, a problem that first appeared April, 2017, but I don't think anyone can definitively point the finger at TB - incompatible security programs are a more likely culprit.

'</blockquote> ...incompatible security programs are a more likely culprit. </blockquote>

I'm using Linux, not Windows but (unless there's some security in TB, FF or Linux I don't know about) using no AV or firewall made no difference to mail delivery. A fresh install of TB (under Mint "Rafaela") with a clean profile worked a few times sending mail then refused. Again, AV and firewall weren't used for test.

I'VE FIXED THE PROBLEM I've been having the same problem. Receiving emails from my hotmail account but NOT being able to send. I have two hotmail accounts. With little help from Microsoft tech support or any working solutions from Thunderbird support - I'VE JUST FIGURED IT OUT AND IT WORKS!!! Leave SERVER SETTINGS at it was imap-mail.outlook.com Port 993 TLS at the bottom of the list 'OUTGOING SERVER SETTINGS' edit settings to use smtp.outlook.office365.com Port 587 STARTTLS and make sure that under 'ACCOUNT SETTINGS' relfect same smtp settings - if not select from the drop down list! All is working fine both in and out now - Hope this helps others :)