delete an unsent read receipt for account that doesn't exist

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I accidently said to send a read receipt for a message
that was received on an email account that no longer
exists.

Outlook is now constantly trying to send the receipt, but
it won't go through since the email account is not in
outlook anymore, so a dialog box is popping up all the
time. I can't see the read receipt in order to delete it.

Thanks much for any help