A useful feature of many mail systems (including Gmail, Microsoft Exchange, Postfix) is the ability to append a ‘+something’ to the user part of an email address. This is known as ‘Plus addressing’ or ‘subaddressing’
This is known as the ‘detail’ part as defined in an Internet standards document called RFC5233 – Sieve Email Filtering: Subaddress Extension.
Uses
Sorting mail
If you receive a lot of mail then you can save a lot of time by having your mail system deliver it into folders. so that, for example mail can be sent to ‘john+subaddressing@apppleseed.org.uk’, where it will fit with other mails on the subject.
Differentiating accounts
Many systems on the internet now use your email address as the name of your account. Although you should always use a different strong password for each account, and a password manager, having the account name being different in each case adds an extra level of protection.
Suppose some criminals break into the computer system for the online shop where your buy your buy your favourite pretzels and steal all the user names and passwords. If you had given your address as ‘user@example.org’ and used the same password for your bank, or any other site, then the criminals can use your details to break into the other sites, or sell the details on the dark web for others to do the same. If, instead you had used ‘user+pretzels@example.org’ for your account, and did not re-use this username then it will not work on any other site.
In the event of a large theft of user data, being able to track which accounts later appear for sale on the dark web could enable investigators to where the leak of your personal data originated.