Verification that my SSRS subscription was emailed to a valid address
The easiest way to do it is to set up a Reply email: address when you you are configuring database mail.  This will send the bounce back messages to that email account. 

UPDATE:

The bounce-back for invalid email addresses will go to the FROM address.  The Reply-To address is used only by an email program when a user tries to reply to the message.  Change the FROM email address and see if you get the bounce-back your inbox.

If you get the bounce-back to the from address you can setup a mailbox to collect these messages.