After update event triggered by changing tab

[Edgar_]

The issue I have is that the when the user clicks on tab page which contains the main form the afterUpdate event of the main form is triggered before the subform is filled by the user.

Those events were triggered before your user entered the subform if the main form was already dirty. The main form can not trigger the After Update or After Insert events exiting a subform.

As for the tab control, the thing to watch out for is the auto focus. Clicking on a tab shifts focus to the first control on that tab. If the first control is a subform, clicking the tab will enter the subform. As previously mentioned, if you enter a subform while the main form is dirty, the main form record will be saved. To prevent this, add a Before Update or Before Insert event to the main form.

The notification should be sent only after entering records on the suborm and when the users either close the form or save the record on the main form.

The main form afterUpdate event should trigger an email notification

No, you should be using the Close or Unload events to send the email. And if you have a save button, then that button is the smart place to send the email.