Take a look at MailHub:
It allows you to set a reminder for emails, so it might at least approximate what you're looking for.
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Thanks for the suggestion. I looked over the manual for MailHub and it can be made to do what I want. However, the process would take longer than the method I use now (simply moving the sent message from the SENT folder to a folder named AWAITING REPLY--CHECK DAILY. If I needed to deal with a lot of messages, then MailHub might well be the way to go. However, I generally add only 5-10 messages a week to the AWAITING folder. Again, thanks for the help.