A "550" response being returned from an email server means there is an error in what you are attempting to do.
The most common 550 errors sent by our email server are:
550: User does not exist - which is returned in response to people attempting to send emails to an email address that doesn't exist
550: Authentication is required - when you attempt to send email to an account that doesn't exist on our system but don't authenticate first.
There are other 550 errors you may come accross too, but in all instances a 550 error is a genuine message from the email server letting you know there is something wrong with what you are trying to do.