Update thank you to Johannes in the comments.  This is called Email Sub-Addressing.

I was catching up with my brother, and he showed me a pretty cool trick with gmail.

If you mail mark-liu@gmail.com (not his real email address) with mark-liu+test@gmail.com - the email would arrive at his inbox.

Not to be outdone, I tested this with Outlook, and sure enough.  If you mail: john-liu+test a outlook dot com they'd arrive in my personal mailbox.  You can do anything after the + character.  +spam, +newspaper, +test1 etc etc.

I have no idea what this feature is called - for Gmail it seems to be called "Creating Multiple Custom Email Addresses with one Gmail account".  May be this is called Email-Alias? 

So this is just a short blog about an Outlook version of this feature.  This works for Hotmail as well.

What is this good for?

• Test throw away emails, e.g. testing your registration process code
• Spam emails and figuring out where they came from
• Easy to set up filter rule to get rid of emails you don't want
• You should definitely have a john-liu+spam alias for signing up to newsletters.

Notes

• Doesn't work for Exchange or Office 365 email addresses.