I have been having this problem quite frequently these days. These are three common reasons why Google thinks Gmail users are receiving someone else’s mail.
Your address is similar but has more or fewer dots (.). Because Gmail doesn’t recognize dots as characters within usernames, adding or removing dots from a Gmail address won’t change the actual destination address. Messages sent to email@example.com and firstname.lastname@example.org are all delivered to your inbox, and only yours. Google Apps does not have this feature and interprets . as dot.
Your address isn’t listed at all. If you don’t see your email address in the To: or Cc: fields of the header, the sender has mailed you a ‘blind carbon copy,’ or Bcc:. The Bcc: field is not displayed in the header of received messages. This means that you won’t see your email address at the top of any message you receive as a blind carbon copy.
You’re receiving delivery failures for messages you never sent. Many spammers use software applications to generate random lists of email addresses at common domains, based on words in the dictionary. Spammers then use these lists to send illegal mass mailings. This practice is called ‘dictionary spamming.’
( via. Gmail Support.)