There are a few things that could cause this:
- You could have the wrong SMTP port specified.
- The sender of the message does not have the required permissions necessary to complete delivery.
- You are trying to relay your mail through another SMTP server, and the server does not permit you to do so.
- The SMTP server is not on your host machine and the web host is blocking you from accessing external SMTP servers.
- The IP address for the receiving server is blacklisted as SPAM (probably unlikely, though possible in a shared hosting environment where many domains share an IP address-i.e. someone else on your IP was spamming).
- The domain name for the receiving address is not fully propogated (which would only be an issue with a new domain)
I'm sure there are other possibilities too that I'm overlooking. Double-check your SMTP settings and make sure the ports, etc. are correct and if the error persists, you may want to contact your web host and tell them you're unable to send mail through SMTP.
If you have access to a different SMTP server, give that a try and see if you get the same error.