There are no Thunderbird mail accounts. So offering to help is limited by your request being based on a false premise. Thunderbird is a mail client and aggregator collecting mail using the IMAP and POP mail protocols from existing mail accounts with internet email providers such a Gmail, Yahoo, GMX, Yandex and Outlook among many many others including ISP provided accounts that it is configured to connect to. I would personally configure the gmail workspace account in Thunderbird and continue working as I did in the past. Email is email from that perspective. The Thunderbird client would now have two accounts and all mail would just appear, so nothing would be lost. This would work as would any forwarding arrangement with the provider so long as the provider does not close the email address. Depending on the arrangements with the previous provider it might be the account just closes and mail is bounced by them when your "client" stops their monthly subscription to whoever is supplying the service now. But that does not address your issue as you express it. Google has a support article on how to do what your trying to do in the Gmail account. https://support.google.com/mail/answer/21289?hl=en&co=GENIE.Platform%3DDesktop Fundamentally This is not a Thunderbird issue. It is a google workspace configuration issue. Unless Thunderbird is running in the background anything done in Thunderbird to "forward" incoming mail will simply not happen, so I am not going into an detail about using filter to forward emails, because fundamentally there are much more reliable ways of doing it.