What is an email alias format (plus-addressing)?

Plus addressing allows a user to add a plus sign and tag to their address in order to create on the fly aliases. For example, captain@tidalmail.com can sign up to a newsletter as captain+newsletter@tidalmail.com and to a harbor report as captain+harbor@tidalmail.com.

Mailbox providers that support this treat everything after the plus sign as routing metadata. Think of it as flying different colored flags on the same ship. It helps with filtering, tracking list sources, and identifying which signup leaked if spam starts arriving.