Well, you could try the translate function and replace the space character with the period.
translate(Insert Field or Group, " ", ".")
Use the Insert Field or Group button to navigate to your field with your name in it.
So you know, this has the potential to be fragile, depending on your environment - if someone has a name with multiple spaces, but only one is replaced with a dot in the email, you may have issues - Mary van der Haven would become Mary.van.der.Haven - just something to be aware of if spaces are not consistently replaced with dots in actual email addresses.