Determines if all changes will be automatically updated.

A Boolean value.  

object.AutoUpdate [= value]

The AutoUpdate Property syntax has these parts:

If you don't want to think about updating contents of Mailbox file with your changes, you should set this property to True. Whenever you make a change to mailbox, it will be automatically updated. However - you should think about this more carefully. If you're working with large mailboxes, and you change messages inside frequently, result will be that on each change, no matter how small or big it is, full mailbox will be recreated with your changes inside.

Is this really worth? It is, on small changes. But, imagine doing something like this:

Msg.Text = "Hello" & VbCrLf
Msg.Text = Msg.Text + "This is line 1" & VbCrLf
Msg.Text = Msg.Text + "This is line 2" & VbCrLf
Msg.Text = Msg.Text + "This is line 3" & VbCrLf
Msg.Text = Msg.Text + "This is line 4" & VbCrLf
'...

Do you have idea what the result will be? Yes, for each line full mailbox will be recreated. Is this what you really wanted in this case? I suggest setting AutoUpdate property to False, making changes, and calling Update method manually whenever needed.

Good idea would be to set AutoUpdate to True just before finishing your work, to be sure that mailbox file is updated with your changes.