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I've seen people list email addresses at Gmail with "+" within the recipient name.

Here's an example of what I mean:

recipient+name@gmail.com

That said, if you attempted to create a Gmail account with "+" recipient name, you would get this error message:

Please use only letters (a-z), numbers, and periods.

Is the "+" simply read and removed by the mail server, and if so, is this specific to Gmail, or is it universal to all email addresses/servers? Meaning my guess is that people to this for readability, but I've always been puzzled by it, since it appears to not be possible to create an Gmail account using a "+".

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"sub-addressing" and some other providers allow for - (minus) instead –  Eight Days of Malaise Apr 26 '12 at 1:17
    
Here is a technique I use to filter certain notes to myself to appropriate labels using "plus addressing". –  Al E. Jul 27 '12 at 12:59
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4 Answers 4

up vote 11 down vote accepted

Plus sign is GMail's delimiter for aliases, which many people use for filtering and tracking spam.

For example, you want to make sure that your bank's emails are all legitimate so you'll register your email with the bank as johndoe+mybank@gmail.com and then create a filter that deletes all emails that don't match from:MyBank AND to:johndoe+mybank@gmail.com. Alternatively, when you register with a shady site you can provide your email as johndoe+shadysite@gmail.com and then create a filter to delete everything that is sent to that address.

However, beware that some sites have poorly-written validators on allowed addresses, which block emails with plus sign, which is actually an allowed character per RFC 5322. Google doesn't allow it simply because it's a reserved character in their system.

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plushaters has a rant about sites with poorly-written validators, with links to better validators. "plussed addresses" work the same with lots of other email providers. –  David Cary Apr 26 '12 at 18:09
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The plus sign is not simply a Gmail feature, but one of the valid characters in email addresses as specified by RFC-5233:

http://tools.ietf.org/html/rfc5233

It's true that many email validation tools don't take this into account. RFC3696 details techniques on how to properly validate email addresses:

http://tools.ietf.org/html/rfc3696

Of note is that the following characters are valid as long as they are not used to begin or end the local part of the address:

! # $ % & ' * + - / = ?  ^ _ ` . { | } ~
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I think only the dot cannot be used for the beggining and end of the local part. Otherwise the others are allowed. From RFC3696: Without quotes, local-parts may consist of any combination of alphabetic characters, digits, or any of the special characters ! # $ % & ' * + - / = ? ^ _ ` . { | } ~ period (".") may also appear, but may not be used to start or end the local part, nor may two or more consecutive periods appear. Stated differently, any ASCII graphic (printing) character other than the at-sign ("@"), backslash, double quote, comma, or square brackets may appear without quoting. –  Tiberiu-Ionuț Stan Mar 17 '13 at 12:40
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Google allows the user to use the plus sign and the period to make variations on their address

  • The plus sign allows to to add any words you like after your base address
  • the period lets you divide your name up anyway you want.

These are great for filtering your emails.

You are right that you can't have plus sign in your base address.

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Adding the + sign to an email address is simply a variation of the same email address, just like the . in Gmail accounts.

The reason why people may use the + sign in their email address with Gmail is to filter their emails. In Gmail, you can filter emails based on who they're sent to, not just who they're sent from. Keeping this in mind, some people utilize this feature to prevent spam, or to better sort their inbox.

Source: I use Gmail, but there's also a help article on Google's website explaining this

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