Hopefully, this can be of assistance to others.
I learned that CenturyLink did some kind of DSL maintenance at the DSLAM in my neighborhood this morning. The Internet connection went out at about the same time and did not come back on by the time I left for work. Upon returning and rebooting my DSL router and all internal routers and servers, I found my out that my mail server and web server hosted internally and port-forwarded from my DSL router were still not accepting connections from the outside. Tech support was of little help except telling me that there was some maintenance in my neighborhood this morning.
From the router status page, it seems to have had a fairly recent firmware version (time-stamped in 2016) uploaded on it. (That wasn't my doing, so I assume CenturyLink did that.) I suspected that I needed to load the factory defaults on the router to fix any lingering firmware upgrade issues. After doing so and re-adding my port-forwarding settings back into the router, my internal mail server and web server began receiving connections from the Internet again and functioned as before.
It only took me most of the evening to troubleshoot and fix it. Thanks CenturyLink!
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