Is it standard practice to block or allow email addresses with a ‘+’ in? - validation

Question

I want each user to register with a unique email address. However some email addresses like GMail allow you to add a + suffix which could be used to register multiple accounts to a website but it all goes to a single email address e.g.
bob#gmail.com goes to bob#gmail.com
bob+1#gmail.com goes to bob#gmail.com
bob+2#gmail.com goes to bob#gmail.com
bob+3#gmail.com goes to bob#gmail.com
bob+4#gmail.com goes to bob#gmail.com
Effectively they can have as many email addresses as they want. This is a problem because my website sees it as 5 separate email addresses but gmail sees it as one email address.
I was thinking of blocking any email addresses with a ‘+' in, but I don’t want to block any valid email addresses. What is the standard practice?

Answer 1

I don't think there is a standard practice on how to handle this, other than not allowing + all together. On the other hand, preventing it doesn't seem to be that useful. It won't take more than a few minutes to create an entirely new e-mail address on some free service if whoever you're intending to block-out really needs it.
It should also be noted that a lot of other e-mail providers also provide subaddressing, but not using the plus sign, but with a hyphen (Yahoo, Runbox, etc.), and attempting to block this out will only cause trouble for anybody just having an e-mail address with a hyphen in it. It's a war that you've already lost.
Besides, if you filter out plus signs, you're essentially not compliant with the RFC3696 standard anymore:
The exact rule is that any ASCII character, including control
characters, may appear quoted, or in a quoted string. [...]
Without quotes, local-parts may consist of any combination of
alphabetic characters, digits, or any of the special characters
! # $ % & ' * + - / = ? ^ _ ` . { | } ~
But you could just strip out the plus part if you insist.
$emails = array('bob#gmail.com','bob+1#gmail.com','bob+hello#gmail.com');
foreach ($emails as &$email)
{
list($identifier, $domain) = explode('#',$email);
list($name) = explode('+',$identifier);
$email = $name."#".$domain;
}
print_r($emails);
The above will give you
Array
(
[0] => bob#gmail.com
[1] => bob#gmail.com
[2] => bob#gmail.com
)

Answer 2

Email ids can contain many characters which would look incorrect to us, I found a good thread here which might answer your query: What characters are allowed in an email address?
Also to find the unique email id, just take the first half of the email id and remove + and . chars and then verify.