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This is very useful if you are using your own domain, and need to migrate from an existing system to FastMail in a staged approach. You can add the domain at FastMail, add the accounts & aliases in that domain, but while the domain is still pointing to the old server, you can setup forwards from the old system to the same names, but hosted at the FastMail server via the forwarding tunnel approach above.
For example, say you have the domain @mywork.com with 100 users already at an existing provider, and you want to setup a FastMail business account and move the existing users to FastMail. You want to move one user at a time starting with the user email@example.com.
So you signup a FastMail business account, and add the domain @mywork.com to it and activate the domain by changing the MX record for active.mywork.com to point to us. You then create the user firstname.lastname@example.org. Now because the domain @mywork.com still points to the existing server, any email sent to email@example.com will go to the existing system. To actually forward it to FastMail, you setup a forward at the existing system so any email arriving for firstname.lastname@example.org is forwarded to email@example.com. Now any email that arrives for firstname.lastname@example.org at the existing system will be forwarded to the email@example.com account at FastMail.
You can then repeat this process for each existing account, and when all accounts are done, you can switch the MX records for the domain to point to our servers and the transition will be complete and seamless.