Can I add an alternate email address with a different domain?
Not on your own. By default, alternate email addresses must share the same domain as your primary support address. Plain support can set up an address on a different domain for you.
How alternate addresses work by default
Plain supports up to 25 alternate email addresses per workspace, such as billing@ or security@. In the self-serve setup, every alternate must match the primary domain. If your primary address is help@acme.com, billing@acme.com is a valid alternate but billing@acme.io is not.
Set up an address on a different domain
Contact Plain support with the address you want to add and your current primary address. Plain will send you the DNS records to add to your domain, then configure the address once they are in place.