What is a sub-address?

A sub address is the tag added through plus addressing. It tells the receiving system how to treat or categorize the message.

A historical footnote adds flavor. The qmail mail server popularized the use of a hyphen as the sub addressing separator which is why you may still see forms like captain-news@tidalmail.com on older systems.

It is metadata built directly into the address and requires no special setup. Behind the scenes, many mailbox providers store the tag but route the message to the main mailbox.