Among all SMTP reply messages, error 550 is surely the most common. It generally comes with a side-message like "550 Requested action not taken: mailbox unavailable" or "550 5 2 1 mail from refused spam site".
Simply put, a 550 error code means that your SMTP server isn't able to deliver the sent email to the user because his mailbox does not exist: either the customer has entered it wrong (a typical example is something like firstname.lastname@example.org instead of email@example.com), or the account has been deactivated and replaced with a new one (but you didn't know it).
In any case, you must remove this address from your mailing list: keeping sending messages to it will just cause other 550 messages. This is not simply annoying; it is dangerous as well for your deliverability. In fact, it implicitly suggests to ISPs and reciving systems that you don't really care about your list hygiene - just like spammers: so with time you could be penalized and see your delivery rate decrease.
Of course the best practice here is not simply to erase the address that provokes a 550 error code, but to replace it with a new one whenever possible. How to do that? If you have another way to contact your user - a phone number, Twitter, Skype... - just send him a line to apprise him that his current email account is wrong, and to ask him a functioning one.