Below could be the possible causes for this problem
1. The SMTP server does not allow relaying unless you are sending email from a trusted IP or use proper authentication.
2. Microsoft Exchange server: server is setup to block SMTP relaying.
3. Some servers allow you to send email internally but not to external email addresses.
4. Some SMTP servers require the Email User to login and check their POP3 email at least once on the given IP before they let that user send SMTP mail from that IP. Once the POP3 email is checked using a valid user/password, the computers IP is then automatically added to a safe list on the server. This is another way to prevent spam.
5. Relaying denied is a common safety measure used by SMTP servers when a certain email address and its mail server are put on a blacklist for email relay spam violations. Many honest mail servers get put on this list because someone broke through and used their server to relay spam. There are various blacklists available and SMTP servers may subscribe to none, a few or many of these lists. Hence you may be able to send email to a blacklisted address through one SMTP server but not through another.
Here are some solutions which could help in sorting this problem,
1. Make sure with your email administrator that your email server is setup to allow SMTP relaying from trusted local internal IP addresses.
2. Make sure that your computers IP address is on the list of trusted clients on the email server.
3. Some email servers may allow relaying only if you log in and check email at least once from a given IP address. In this case, you can check your POP3 email first before sending SMTP email from a new computer.
4. Some email servers allow SMTP relaying only if you log in and check email within the last 1 hour per day and so on. In this case, you should use the pop3 authentication method to send email.
5. Some servers require you to authenticate before sending email. In this case, use the SMTP or java authentication method.
6. Microsoft Exchange: make sure that your exchange server is configured to allow SMTP relaying either for trusted addresses or for users that are authenticated.