To send messages in Fastmail from an address serviced by another email company, there needs to be a connection between Fastmail and the other provider.
Many email providers require other services to connect with them using modern authentication, or OAuth2. If you have connected an address from another provider to Fastmail using our guide here, we're currently using OAuth2 to connect with the other account. No further action should be needed.
If you've previously set up sending through an external email address, you'll need to make some adjustments when the other provider requires OAuth2.
To connect with your other email address using OAuth2 and continue sending from this address in Fastmail:
- Go to the Settings → My email addresses screen and remove the existing external address by clicking the Edit button, then Delete.
- Go to the Settings → Migration screen and select your email service provider from the top of the screen, then click Sign in. If your provider is not listed, select Other (IMAP/CardDAV/CalDAV).
- Enter the username and password (or app password) for your account with this provider.
- Select the checkbox next to Set up sending from your email address via your Fastmail account. Note: To only send from this address, and not receive any of its incoming mail to Fastmail, or import its other data, de-select the checkboxes next to all other options.
- Click Do it.