Plus addressing, also called subaddressing or tagged addressing, is a feature supported by Gmail, Outlook, and many other providers that lets you insert a plus sign and an arbitrary tag into the local part of your address. Mail sent to name+tag@example.com is delivered to the same inbox as name@example.com, with the tag carried along for your own use.
The primary benefit is organization and tracking. By giving a unique tag to each service you sign up for - name+newsletter@, name+shopping@ - you can filter incoming mail automatically and, crucially, discover who leaked or sold your address, since any spam arriving to a specific tag reveals exactly which signup was the source.
For senders and list managers, plus addressing has an important normalization implication. Because name+a@ and name+b@ reach the same mailbox as name@, they are effectively the same person. Treating tagged variants as distinct contacts inflates lists and can cause the same recipient to receive duplicate mail, so normalization often strips the tag when deduplicating.
There are limits. Not every mail system supports the plus convention, and some signup forms wrongly reject addresses containing a plus even though it is valid. Some users also exploit tagging to create many apparent addresses from one mailbox to abuse free trials, which is why fraud-conscious systems normalize tags before counting unique accounts.