I might also add, that I have two different Roadrunner email accounts which I have profiles for in my Thunderbird program. One still checks for mail just fine. The other won't even check for it when I click on "Get Messages", as it used to. Upon opening Thunderbird, the message bar across the bottom states, "downloading message 1 of (insert number here)", and I can see the progress bar begin "loading" across the bottom, but it barely starts and then stalls, making no more progress, and the "downloading message 1 of..." message in the message bar goes away, though the stalled progress bar remains.

I am only having this issue on one email profile, not both. I have not changed any settings on either profile, so I don't understand why this has just started happening for no apparent reason.

Is Road Runner a POP email service? It's not unknown for POP servers to become 'stuck' on a particular message -- sometimes because of its size or its attachments, sometimes because of characters in the message itself. The usual remedy is to leave Thunderbird closed and log in to that account via webmail instead. Find the message that may be causing the problem and delete it. It should be the next message waiting to be downloaded after the last one you downloaded successfully to Thunderbird.

Thanks for your reply. I very much appreciate your willingness to offer help. :)

After posting here, I continued to think the problem over. It then occurred to me that there was something very familiar about the circumstance itself, recalling that sometime in the distant past this had happened to me before. It seemed to me that I had been told long ago that sometimes a message can become corrupted in some way, thus being incapable of being downloaded to my computer. So I came to the same conclusion you did, and did indeed do exactly the same thing that you suggested. I went to webmail and deleted the oldest email there, as I know my messages are downloaded in time stamp order. Sure enough, all the messages that had stacked up "behind" the troublesome one, were then downloadable by Thunderbird. So that solved the issue for me, but I failed to come back here and post that the issue had been resolved already.

Thank you again for taking the time to respond to my problem. My hat's off to you! :)