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Here is a fully automated version. It will automatically send the reminders whenever a cell in column "E" is changed or the worksheet is activated. Once sent, a date stamp is entered in column "H". This not only marks when the reminder was sent but also prevents multiple reminders from being sent.
Two new columns have been added to the worksheet: columns "H" and "I". The former column holds the reminder date stamp. The latter column holds the email recipient's address. Reminders are sent only when the date is in the current month, no reminder has been sent, and an email address is in column "I".