Error 550 Relaying not allowed

RESOLUTION

This error message is coming from the mail server, not from Outlook Express. This error means that the SMTP (outgoing) mail server did not recognize you as an authorized user and rejected your e-mail message. Most ISPs (ISP stands for Internet Service Provider, and this includes GCI) configure their mail servers to recognize only those computer IP addresses that are on an accepted list. If you use multiple ISPs, or just switched from another ISP, you are likely to encounter this problem. The solution is to change setting of the SMTP server for that mail account under your e-mail application: