Ok I made some assumptions:
Only one email per account even if there are multiple activities, and I ended up using outlook.
There are some items to edit in the code (I made red what you need to update), I assumed your columns in the sample match the actual sheet with real data, so keep those in mind. I did have to use column F to mark the reminder as sent to avoid you getting the same reminders day after day.
You have to update the code with your real email address, and there is a ' (which comments out lines) in front of .Send that will need removing once you are satisfied that it works as desired.
The sub is called auto_open, which will make it run whenever you open the workbook. You could also put the code in the workbook_open event instead if you prefer.
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