Great, you've decided to migrate your business to FastMail. Migrating email accounts from one system to another can be a dangerous process. To make it is easy as possible, and to avoid any chance of lost emails, we've setup many tools to make it as easy as possible.
An important help article outlines all of the necessary steps to perform: https://www.fastmail.fm/help/business_migrate.html
Another one of FastMail's hidden gems is the forwarding tunnel.
The forwarding tunnel allows you to forward email from an existing service to an account with the same name at FastMail. e.g. Say you have the account firstname.lastname@example.org at an existing service. You want to create your account as email@example.com at FastMail.FM, but how do you forward from the old firstname.lastname@example.org account to the FastMail.FM email@example.com account while the DNS still points to the old server? This is what the forwarding tunnel solves.
It basically gives an extended email address syntax that you can forward email to, which will deliver to our servers, and then decode to the account name. There are 3 encoding options because some providers are limited by what characters you can put in an email address to forward to.
To forward email to the FastMail account firstname.lastname@example.org, you can forward to any of these 3 addresses
By utilizing the forwarding tunnel, you can ease the transition from an old email system as it migrates to FastMail. You can setup all the email clients to use FastMail and each business user will receive all their email from one place.