This is an end-user level question so you'd best ask it where there are people who do end-user support:
http://answers.microsoft.com/en-us/office/forum/outlook
Before you post, I recommend you contact your ISP (Internet Service Provider), as the messages you're seeing recommend. Due to SPAM and malware, ISPs do lock out certain domains and address types. You need to find out whether this is the case, because there's
nothing anyone at Microsoft can do, and nothing Outlook can do, if your ISP is blocking this kind of email account designation.
Cindy Meister, VSTO/Word MVP