Here's some code I've just used

Open App.Path & "\ReminderLetter.html" For Binary As #1
Put #1, , strHtml
Close #1

This opens "Reminder Letter.html" which is in the same folder as the App (App.Path)

Put #1 then replaces the existing ReminderLetter

I then added ReminderLetter.html as an attachment

Hope it's useful.