Help please with auto send email VBA, it was working but isn't now

[nje]

I have attached a sample file with all the codes.
I have an 'auto send email' (thanks to a forum user) that works when you open the sheet.(although I would really like it to send when you open the book)
It was working in another sheet but when I have adapted it, it has stopped sending on opening . I have put a macro button on the sheet assigned to it and when pressed it works.So it is the just 'auto' part that has has stopped.
Can anyone help me to see what I've got wrong please?
Whilst I'm on, I will have about 10 sheets in the book and would like to run the auto send from each sheet (different data, but the dates will be in same columns)
Would I just put the same code into each sheet to get it to work on all of them, or would that be a problem?
Thanks

P.s. Could anyone also explain to me what this bit of the code means, so I can learn to understand the code a bit better? For i = 2 To lngNumberOfRowsInReminders
Thanks again