Hi, you will need to provide more information in order for people to help you. Thank you. Version information for Thunderbird, platform you are working on ie. Windows 11 POP/IMAP server & SMTP server settings with the username/password starred out eg. ****** Is there a full error message ? if so, what is it ? Server Type : POP Mail Server Server Name : outlook.office365.com Port : 995 Username : ********** Connection Security : SSL/TLS Authentication Method : OAuth2 You will find this in Settings->Account Settings->Server Settings (in the left hand window, under your email account name)