If the basic variation of the SMTP 550 error – “550 requested action not taken mailbox unavailable” – is displayed, then the problem is usually that the intended target address couldn’t be reached by the SMTP server. In such cases, your first look should be at the address itself – it’s possible that a simple spelling error or typo slipped in when the e-mail was created. In many cases, though, the problem is on the recipient’s side. The message is typical if the recipient’s address isn’t temporarily available due to technical errors. The same generally goes for when the contacted e-mail account has been deactivated due to inactivity or deleted.
If you can reach the other party via another channel, you should do so. This way, you can make the problem known or find out the new, current (and valid) e-mail address. If you don’t have this option, then there’s nothing else to do other than try the sending again later. If the recipient was only temporarily unavailable, then the new attempt could be successful.