Using a plus sign in an email is a useful tip to know. It helps me manage accounts I use on the internet and is also incredibly useful for my day job.
Plus sign in email
Say I have an email at example.com.
mike@example.commike+spam@example.com
mike+bt@example.com
mike+testuser@example.comThree reasons why this is useful
Filter out spam
You want that 10% discount but aren't that interested in the newsletter. You need a one off email to sign in to that fun game service. You know you'll get some spam. By using the first example email you can have a filter set on your inbox so that those emails to that address never hit your main inbox. If your email gets sold any new emails will also be filtered out keeping your focus on the important emails.
Specific emails for companies
You may want to have specific emails for certain companies. This could help with security (not using the same email everywhere) or help you group emails together. The example here uses 'bt' but you could use something like 'bills' or 'household'.
Testing
At work I need to do regular testing - this also involves getting emails. By using the plus sign I can create multiple users and still get the email flows. There also isn't the risk of using a domain not in my control and ending up with test or live emails going to the wrong person.
Summary
Three examples of how the plus sign in the email can be useful and I am sure you can imagine plenty more. It enables labelling of the emails when you sign up to something helping preemptively keeping your inbox clean.
Overall the final testing example is the one I use all the time but the plus sign comes in useful many times.