This is an old feature (or actually lack thereof) of Gmail that I think can be exploited for use in avoiding spam when subscribing to mailing lists and when sharing email addresses. Use dots! Apparently, with Gmail, it doesn’t matter if you send to username@gmail.com or user.name@gmail.com–it goes to the same inbox.

Am I receiving someone else’s email?

Gmail doesn’t recognize dots (.) as characters within a username. This way, you can add and remove dots to your username for desired address variations. messages sent to your.username@gmail.com and y.o.u.r.u.s.e.r.n.a.m.e@gmail.com are delivered to the same inbox, since the characters in the username are the same. - Gmail help

So why is this good? You can actually take advantage of this by giving different addresses for different situations. Say, you can give username@gmail.com only to your friends, for personal correspondence (true, this can be spammed, too but by giving out your address to limited people lessens the likelihood). But for mailing lists, you can subscribe as user.name@gmail.com. Then if you prefer, you can create filters that automatically move emails sent to user.name@gmail.com into the archive, and perhaps add a label, so they don’t appear in your Inbox view.

You can even use this to subscribe to a service several times over–pretty useful when signing up for limited trial or free service that require unique email addresses.

You can also automatically assign labels to emails by asking people to send to username+label@gmail.com, but some mail systems don’t treat the “+” sign properly and this might cause errors.

And so you can send using the email address with dots, you can use Gmail’s “send as” feature to change the “from” address as needed.

I actually use my Gmail account as my universal inbox. I forward all my other emails to my main Gmail address, and use the “send as” function to change the choose among my pre-set sender addresses as needed.