The best way to do this is at the server. Go to the website for account 1 and look for options to set up rules or filters to forward or redirect messages to account 2. In many cases, you could also go to the website for account 2 and set it to collect messages from account 1. Do one of these - not both! You ''can'' do this in Thunderbird, using its own Message Filters, but then this would only work while Thunderbird is running. And if Thunderbird is running, you can see all the messages anyway, so you don't gain much. https://support.mozilla.org/en-US/kb/organize-your-messages-using-filters