There's an article that lists the general error codes:
https://community.marketo.com/MarketoArticle?id=kA050000000KytYCAS
There are also a ton of articles on the public interent if you search for SMTP Email Error Codes
The 5.1.1./550 is a general purpose email invalid code letting you know that the receipients email box is not avaiallbe. It could be that it never existed, or it could be that the person left the company and the box has been closed.
You need to develop a process to clean these up, because repeated mailings to an unavialble mailbox could get you blacklisted.