'''Description:''' I have a gmail account "[email protected]" which is already being used by Thunderbird desktop client. Recently I have added an alternative email to "[email protected]" with my custom domain; "[email protected]" [https://support.google.com/google-ads/answer/1752343?hl=en Google Alternate Emails] Both email addresses are using same inbox. I can login from browser via 2 different emails with same password. I can write a new email and Gmail web UI let me choose between these 2 email sender addresses. '''Reason: ''' this custom domain has a custom server without a mail server. Instead building a mail server, we choose to use it with online services, which is already working perfectly fine up until this point. '''Problem:''' It fails when I try to add "[email protected]" in Thunderbird. [https://support.google.com/mail/answer/7126229?hl=en#zippy=%2Cstep-check-that-imap-is-turned-on%2Cstep-change-smtp-other-settings-in-your-email-client Gmail's IMAP/SMTP settings guide ] I get the error "Incoming server already exists", which is true and in this case shouldn't be an error but how it's suppose to be. Right? . How can I succeed to add an email address to share the same "Incoming server" with already existing email in Thunderbird?