Usually, you set up forwarding from one email address to another email address via the webmail account so basically sending from one server to another server. Then you only need the one mail account in Thunderbird for the email address on server that is receiving all the emails. It would connect to server and download everything for that email address and everything else forwarded from the other email address. You can do it in Thunderbird, but it means downloading all your emails to one account email address and then auto forwarding all of them back to another email address on another server just to download them again into another mail account email address. As you already have the emails downloaded I'm not sure what it is you are achieving. You can have more than one mail account showing emails from both email addresses. You would need to create a filter to do it. Obviously, this only works if Thunderbird is running. Menu app icon > Tools > Message Filters Make sure it is for the correct email address mail account which currently receives emails that need forwarding. click on 'New' Give filter a suitable name eg: Forward emails Select 'Getting new mail' Select 'Filter before junk controls' select 'Match all of the following' Select option 'FROM, TO,CC, Bcc' Select: 'contains' Enter your email address Perform these actions: Select 'Forward Message to' and enter the email address of other account' click on OK Then make sure the checkbox to enable the filter is selected. Run selected filters on 'Inbox for email address' Click on 'Filter Log' make sure log is enabled by selecting checkbox 'Enable filter log' You could test this immediately assuming the mail accounts are created.- click on 'Run now'.