Martin said, "...the problem is probably with your mail server."
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Yes, I believe that's the case. They must be blocking the messages. I'm not sure why. AT&T usually notifies me when action like this is taken. I do not recall advising them to do this.

I'll investigate. Thank you, Martin and Hack, for your attention to the matter. At this point, I'll keep a careful watch on the User CP to track thread responses. That seems to work rather well and is a great Forum feature.