SMTP error codes, meanings and what to do

I made this email error code list because of a number of bounce backs a company was having with their mail list. Unfortunately they hadn't been keeping the list up to date an were having 510/511 and 554 errors as they were using a mail client to send the emails and had sent them a little to often. So to save them and myself time I create this list of errors, so they could see what the bounce back error was and look it up. If you want a printable version I have a google doc version as well as a PDF version. If your looking for great email diagnostic tool MX TOOLBOX is a great resource.