Email Subaddressing

Some mail services support a tag included in the local-part, such that the address is an alias to a prefix of the local-part.

Usually, the characters following a plus and less often the characters following a minus, so fred+bah@domain and fred+foo@domain might end up in the same inbox as fred+@domain or even as fred@domain.

As an example, the address joeuser+tag@example.com denotes the same delivery address as joeuser@example.com.

RFC 5233 refers to this convention as subaddressing, but it is also known as plus addressing, tagged addressing or mail extensions.

This can be useful for tagging emails for sorting, and for spam control.

Addresses of this form, using various separators between the base name and the tag, are supported by several email services, including ...

• Andrew Project (plus)
• Runbox (plus)
• Gmail (plus)
• Rackspace (plus)
• Yahoo! Mail Plus (hyphen)
• Apple's iCloud (plus)
• Outlook.com (plus),
• Proton Mail (plus),
• Fastmail (plus and Subdomain Addressing),
• postale.io (plus),
• Pobox (plus),
• MeMail (plus),
• MMDF (equals)
• Qmail and Courier Mail Server (hyphen)
• Postfix and Exim allow configuring an arbitrary separator from the legal character set.

https://en.wikipedia.org/wiki/Email_address#Subaddressing