I am in favor of adding this feature. However, personally I think it is best to send a correction as a new email, and clearly indicated as a correction. When an error important enough to need a correction is found, it seems to me as important, if not more so, for it to go out to the people who HAVE ALREADY opened the erroneous email as to send the correct mailing to those who never opened it. Where I would see more the importance of being able to change the text of the re-send is in a case, for instance, where you are announcing an event that's coming up in a week. If you send it to non-openers 3 or 4 days later, you want to change it to say "there are 4 days left to [event]!"
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