I know Outlook comes in several varieties, so my setup may not work for you, but I use outlook.office365.com and port 587 for SMTP

I don't have office 365 but tried it before and it didn't work so I changed it back to what I said and port is 587 (default) in both outlook which works and thunderbird which doesn't work.

It was just a guess, as I don't have office365, either.

David-Do you know anyone else who can help me with this problem?

You appear to have the correct setting. There are others on this forum who may help if they see the post.

I am having this problem too.

"Sending of the message failed. The message could not be sent because connecting to Outgoing server (SMTP) smtp.office365.com failed. The server may be unavailable or is refusing SMTP connections. Please verify that your Outgoing server (SMTP) settings are correct and try again."

I am having no IMAP problems and I receive a lot of email to this account.

I do not use this account to send very often, but it started happening in the last 2-3 weeks. I have tried all combinations of smtp.office365.com, smtp.live.com, and smtp-mail.outlook.com. Starttls and ssl/tls. Oauth2, Normal password, and encrypted password. Default port of 465 or 587 with all combinations.

No combinations have worked, I get the same error message on all. I have a VPN, but it is disconnected.

outlook.office365.com is the incoming IMAP or POP server. The correct outgoing server is:

smtp.office365.com, port 587, STARTTLS security, OAuth2 authentication, User Name = email address.

https://support.mozilla.org/en-US/questions/1417298#answer-1588949

The correct IMAP settings are:

outlook.office365.com, port 993, SSL/TLS security, OAuth2 authentication, User Name = email address.

If it still times out, it could be caused by antivirus scanning of outgoing mail, especially if the app is a non-MS one like AVG/Avast, Norton etc.

Hello

Take a look ? https://connect.mozilla.org/t5/discussions/outlook-and-thunderbird/m-p/47664/highlight/true#M17031

All the settings match mail-outlook which works to send so the problem is with thunderbird-perhaps this problem should be given to the developement team. I have received several responses but none solved the problem but thanks to all that replied.

Microsoft publish the settings for accessing your account with them using IMAP, POP and SMTP. They are here https://support.microsoft.com/en-us/office/pop-imap-and-smtp-settings-for-outlook-com-d088b986-291d-42b8-9564-9c414e2aa040

Agent virtuel said

Hello Take a look ? https://connect.mozilla.org/t5/discussions/outlook-and-thunderbird/m-p/47664/highlight/true#M17031

Another person that mistakes a feedback forum for user support. Was there more to this? Probably! Like if they need to extend the timeout period, they need to get rid of the antivirus product that is slowing their system for no security return.

It is actually quite rare for an update in Thunderbird to affect the ability to send. The exception being servers using encryption that has been obsolete for most of the last two decades, and they are still out there.

As Thunderbird has not in the last year or so disabled any obsolete/broken encryption then everyone should be looking outside of Thunderbird for the cause, despite the symptom being observed there. For windows users, and increasingly Mac users the most common failure point is the antivirus and the software firewall it installed with it. One blocks the account setup wizard and the latter blocks this new unknown program for any access to the internet. It has to protect you.

Then most of these antivirus products use self signed certificates to hack your encrypted connections to do their scanning. It is essentially a man in the middle hack. But it is a delicate house of cards and as soon as something changes it is just as likely to just break something. Like getting or sending mail.

As these self signed certificates are unknown to the chain of trust used in the wider internet they have to be added manually to the certificate store in Thunderbird or connections simply fail. So any connection using SSL/TLS or STARTTLS is susceptible to the self signed certificate messing up the connection.

Personally I just do not do any email scanning using only windows defender. It works for me and I do not have to put up with the regular collapse of some mail function caused by the third party antivirus products.

Have Avast but turned it off-then turned off Windows firewall but still cannot send a hotmail except directly from outlook site.

Forgot to say my gmail works fine with Avast and Windows fire wall are both on-only outlook doesn't send