A plus address (also known as a sub-address or "+ address") lets you create new email addresses that are delivered straight to your inbox, with no further configuration. It is called a plus address because there is a plus sign between your mailbox name and the made-up part.
How it works
If your email address is support@cloudient.net, you could ask people to email support+urgent@cloudient.net. These messages are delivered to the support@cloudient.net inbox, so there is essentially no difference between the two.
The difference is what you can then do with them. Because the sub-address appears in the To: header, you can add a mailbox filter to file support+urgent@cloudient.net into a different folder automatically.
Tracking where mail comes from
Another common use is when you register an account on a website. Instead of your usual address, you would use something like bob+amazon@myemail.com. Any messages sent to that address can then be filtered, and if you start to receive junk mail you can see exactly where the address was leaked. That makes it easy to filter such mail to junk, or to delete it, if an address is misused.
Cloudient supports plus addressing for both mailboxes and forward aliases. For related guidance, see our password policy guidance.