Gmail: A Useful Feature

I wish to discuss an interesting and useful feature of several email service providers including Gmail.

Recepient's e-mail address is used to identify the location for electronic message delivery.

Normally email IDs are in the form: some_name@domain.

An excerpt from RFC 2821 says:

An address normally consists of user and domain specifications. The standard mailbox naming convention is defined to be local_part@domain: contemporary usage permits a much broader set of applications than simple "user names". Consequently, and due to a long history of problems when intermediate hosts have attempted to optimize transport by modifying them, the local-part MUST be interpreted and assigned semantics only by the host specified in the domain part of the address.

Like few other mail services, Gmail has a cool feature that lets you specify an additional string following the "local_part" of the email ID. For example, while subscribing to Qualys Vulnerability Alerts email, I subscribed with the email ID as: ptteam.1+qualys@gmail.com.

According to gmail, this address will be simply treated as ptteam.1@gmail.com and the additional tag "qualys" can be used at my inbox, to easily filter out the message arriving from qualys.
Notice the recipient's address.

Similarly one may use other tags while subscibing to other services or mailing lists.

Interestingly, apart from easy sorting of messages on sources, this feature can also be used as a trick to detect spam sources.

How?

Quite simple! - If a spam arriving at your inbox has recepient's address containing your predefined tag, you can easily make out the site from where your email address was leaked to spammers.

So, it's a good practice to add +someTag while subscribing to some online service or while giving out your email ID to anybody.

But,

Sometimes, it becomes impossible to use this feature, when the form based subscription page of some websites treat the "+" sign as an INVALID character in email ID field.

Addresses of this form, using various "separators" between the "base name in local_part" and "tag" are supported by several email services, including Google Mail (plus sign "+" ), Yahoo! Mail Plus, and qmail (minus sign "-")...and more.. let's find it out.