Further info. for RealblackStuff
The problem reappeared. It occurred to me that the problem may be inherent in AVG, rather than a conflict with another operation. So I tested this theory by sending and receiving emails between user1 and user2, while user2 AVG was showing the email scanner as not fully functional, with both mail servers disabled. The emails were successfully sent and received, and the email certification was printed on each email received. Although AVG was showing the scanner not fully functional, with the email servers disabled, everything was working OK. Consequently, I uninstalled/reinstalled AVG and everything seems to be working just fine. If anything changes, I'll post the results. Thanks for your help.