Typos in an email address can cause Email Error 550. Make sure there are no extra spaces or misspellings in the email address you supplied. There's also the possibility that the receiver provided you with the incorrect email address. Also, confirm with them.
Also, double-check that your email address is correct and that your email contains content. Some email servers will reject emails that contain no content.
After some time has passed, try sending the email again.
There's a risk that the recipient's server is down for a variety of reasons. Inform the recipient that the server may be unavailable and request another email address, or try again in a few minutes or hours.
