SMTP reply messages error 550 is generally the most common. It comes with a side-message like: "550 error - Mailbox does not exist" or "Error 550 - Mailbox not Found" or "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: typically it is because the customer has entered it wrong or the account has been deleted or replaced with a new one.
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 just annoying; it is dangerous as well for your email deliverability. Truthfully known, it suggests to ISPs and receiving mailboxes that you don't really care about your list quality - just like spammers: so with time you could be penalised 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. You do this by simply contacting the person to update their details.