Email bounce back 550 5.1.1 recipient rejected
550 5.1.1 means that the recipient email server believes that the email address does not exist at the destination domain, and therefore has no way to deliver it.
Unless you simply made a typo in the email address, this is not something that you can fix, and must be addressed at the other end.
A Spam filter may also be the cause if emails are passing through it and the email address is not registered on it as yet