Problem: SMTP settings of office365 don’t work with Thunderbird. POP and SMTP work so you can receive emails but sending doesn’t work.
Resolution: You need to disable the smtpautorization.
First Install the exchange online management module in an elevated power shell running this command
Install-Module -Name ExchangeOnlineManagement -RequiredVersion 2.0.5
Then import the module
Now you can connect with your account online , you need to be the admin user. You will be prompted to enter your login credentials after running this.
Connect-ExchangeOnline -UserPrincipalName firstname.lastname@example.org
Now you can modify your user account by running
Set-CASMailbox -Identity email@example.com -SmtpClientAuthenticationDisabled $false
You can see if it worked by running this. You will see a table with all the paramenters and SmtpClientAuthenticalionDisabled should be set to $false