What is Custom domains?

A custom domain means an alias can sit on a web address you control, such as one at your own site, instead of the provider's domain. That keeps your real mailbox private and lets you hand out a different alias per signup. addy.io, CTemplar and Disroot each support custom domains. Apple Hide My Email and Apple Inc. do not, since the feature relays through Apple's own address. Check that your provider supports your own domain, and not only a subdomain.