I have seen another person reporting same problem and they also use BT Business which got auto moved over to the outlook385 server. You mention POP. According to this: https://business.forums.bt.com/t5/Email-and-hosting/Outlook-email-migration-to-POP/td-p/88272 "''Microsoft have disabled POP. ''" However, it may well be due to changes regarding the Authentication Method. So POP/IMAP/SMTP may still be working but you cannot use Authentication Method: Normal Password. See https://answers.microsoft.com/en-us/outlook_com/forum/all/pop3-incoming-server-outlookoffice365com-suddenly/0b78850c-0977-48fa-922a-d071b6596ac1 office365 uses Authentication Method : OAuth If you were using an imap mail account. Try altering the server settings. If you previously had a pop account then you cannot simply alter the server settings to use imap. You would have to create a new existing mail account and set up as imap. '''1. '''First make sure cookies are allowed. * Menu icon > Settings >Privacy & Security * Web content * Select: Accept cookies from sites and 'always' and 'until they expire' '''2.''' remove the old saved password - scroll to 'Passwords' section * click on 'Saved Passwords' * click on 'Show Passwords' * Select mailbox://...or imap://... and smtp://... lines * click on 'Remove' If you currently have an imap or pop account then you can modify the server settings using the 'Server Settings' info below. If you previous had a pop account and now you want an imap account then you will need to create a new imap account first. * Menu icon > New > Existing mail account * Enter name, email address and normal password. * Select checkbox to remember password. * click on 'Continue' and Thunderbird will attempt to get configuration settings. If you need to use the 'Configure Manually' button then use the following. '''3. '''Server Settings - Remember - if modifying the Authentication Method then do both Incoming AND OutGoing SMTP. POP settings: * Server name: '''outlook.office365.com''' * Port: '''995''' * Connection Security: '''SSL/TLS''' * User Name: '''full email address''' * Authentication Method: '''OAuth2 ''' IMAP settings * Server name: '''outlook.office365.com''' * Port: '''993''' * Connection Security: '''SSL/TLS''' * User Name: '''full email address''' * Authentication Method: '''OAuth2 ''' Office 365 SMTP settings * Description: full email address * Server name: '''smtp.office365.com''' * Port: '''587''' * Connection Security:''' STARTTLS''' * User Name: '''full email address''' * Authentication Method: '''OAuth2''' Once account created or you just changed the Authentication Method - then when you attempt to connect to server the first time, you will get a pop up from Microsoft asking for email address and normal password to allow Thunderbird access and then a token gets stored in Thunderbird in the same place as the normal passwords. From then onwards the token is used to access the server.