Usually the address is red to indicate that it is not in your address book. I have seen some reports of the red color even on contacts that are in the address book.

Thanks Airmail!

The problem has been solved but I don't know how!

This occurred in replies to the last two messages I received. A gold star was displayed next to the senders' addresses, which indicated that the addresses were in my Address Book. I also checked and they were there. But when I clicked on Reply, the senders' addresses were displayed in red.

I just went back to both messages and clicked on Reply again, and the addresses were displayed in black. Thunderbird had not been closed and reopened; the computer had not been hibernated; no settings had been changed, although I did look at the Account Setting to see if any of the color settings were red. None were. But everything is working properly now.

Thanks again!