You may be aware that recently Microsoft changed the Authentication Method when it comes to using third party email clients eg: Thunderbird. I'm not sure whether you have already done the following but please check anyway. Account Settings must use the following: Server Type: ''' POP''' Mail Server * Server Name: outlook.office365.com * Port 995 * User Name: xxxx@hotmail.com * Connection Security: SSL/LTS * Authentication Method: OAuth2 Server Type: ''' IMAP''' Mail Server * Server Name: outlook.office365.com * Port 993 * User Name: xxxx@hotmail.com * Connection Security: SSL/LTS * Authentication Method: OAuth2 '''SMTP:''' * Server Name: smtp.office365.com * Port: 587 * Connection Security: STARTTLS * Authentication method: OAuth2 * User Name: example@hotmail.com '''Settings > Privacy & Security''' Check under Web Content - * Select: 'Accept cookies from sites' * Accept third party cookies' 'Always' * Click on 'Exceptions' button next to Accept cookies from sites to make sure you are not blocking cookies from Microsoft sites, such as :hotmail.com, outlook.com, office365.com, www.live.com, www.msn.com If Thunderbird stored your hotmail password then it may be located here: '''Settings > Privacy & Security''' * Scroll down to 'Passwords' section * click on 'Saved Passwords' * Click on 'Show Passwords' If you get a 'Windows security' prompt for a password.... When you start computer, you get asked to login to your User Account using a password to see desktop etc. It wants that password. Then you can see the stored passwords. However, it sounds like you are in a position where you need to check things by logging on to your microsoft account and/or hotmail account via a browser. Your microsoft account and your hotmail account need to have the same password. Info at this link says: * https://support.microsoft.com/en-gb/office/how-to-sign-in-to-hotmail-c378938b-c0eb-40c1-b99e-338dd9e776a8 ''Can't sign in to Hotmail or Outlook.com?'' ''Notes: Your Hotmail or Outlook.com password is the same as your Microsoft Account password.'' So get that sorted first. If you found a password in Thunderbird saved passwords then I suggest you try using that first. If you did not find anything or forgotten - on the webpage where you login to webmail there will be a 'forgotten pasword' link which you need to use. Follow instructions to regain access to your webmail account. Once in the webmail account you can modifiy/update password. In microsoft account If you have more email addresses or alias hotmail addresses etc When it comes to 'sending' - smtp in Thunderbird - it can only send using the 'primary' email address, so if you want to send using hotmail email address then make sure it is set up as the primary email address. Once you have all these settings and got the password sorted. Restart Thunderbird When you get the Microsoft pop up window asking for password, type it in and clickon 'Sign in'. You may then get another window asking to allow Thunderbird access to server - click on 'Accept'