Gmail (or in the UK and Germany "Google Mail"; we'll use "Gmail" throughout this document) is Google's free email service. Along with other pecularities, Gmail supports an excellent feature called plus addressing.
Say you have the Gmail address a@gmail.com (and we can safely say that, since that Gmail address isn't long enough to be a valid, registered Gmail address). You use it as your primary mail address, or at least use it a lot. Maybe you want to track what happens to your mail address when you give it to a certain site or program. Maybe you want to make it easier to filter your mail based on where they come from.
Here's when plus addressing comes in. Gmail lets you stick a plus sign (+) before the @ sign, and then type whatever you want - like, say, a+website@gmail.com for email sent from your web site. Regardless of what you choose in between the + and the @, it will be delivered to your usual address a@gmail.com as if you had typed that, but the message will say that it was delivered to, say, a+website@gmail.com.
The practical upshot of this is that you can give out a+microsoftdotcom@gmail.com to one web site, and a+appledotcom@gmail.com to another, and you can see which one sends you mail just by looking at the "To:" address, which makes for easier filtering. If you visit a web site you're not sure is that serious, you can give out a+creepywebsite@gmail.com and create a filter that applies a label to it, or archives it, or stars it, or moves it to the trash, or...
Other email delivery systems implement plus addressing as well, but it's become more widely known through Gmail.
Some email address validators - the parts of regular desktop programs or web sites that makes sure you can't just type anything into an email text field that doesn't even look like an email address - does not recognize the + sign as a valid character to put in an email address. For example, you could try to sign up for a newsletter using a+newsletter@gmail.com, and the sign up form tells you that "the email address you entered is not valid" or something to that effect.
These validators are flat-out wrong. If you want the creators of the newsletter to fix this, send them an email with a link to this page.
The applicible accepted worldwide standards for email explicitly include the + sign as a valid character in an email address. (See: RFC2822, 3.4.1.)
Email address validator writers, if your email address validator does not accept + as a valid character in email addresses, not only are you locking people out from using a good organizational device, but you're breaking the worldwide standard, and effectively validating something else than an email address. Please fix this behavior for the good of your users.
If you have anything to add to this document, or any questions, please contact me - Jesper - .