OK, this question has been answered on this forum numerous times. It was hinted-at above. Most ISP's do not allow the use of foreign ISPs on their domains. They do this as a security measure, primarily to prevent their systems from being used by spammers. As others have suggested, you may be able to work around this restriction by selecting a different port. The default port for SMTP is 25. Try port 587 instead. If this does not work, then you will send your replies via webmail.Not sure about the port number. I'll have to contact the ISP manager about that. It's unlikely that both addresses at different servers have the wrong port. Also, it happens when traveling far more often than at home.