hey Niels - you are changing the spec! 
I agree that Mike’s way is definitely more robust and likely to handle more cases other than the initial three formats presented - but obviously at the price of some complexity.
I like the way Mike first finds the @ then gets the Domain to the right and the User (and perhaps Name) to the left.
I am so used to doing everything in ST these days it does feel a bit strange to see “old school” coding like Mike’s here (and also sometimes some of Carl’s stuff).
one suggestion - look at the lines:
L = LEN(CLIP(LEFT(E)))
LOOP X = A+1 TO L+1
here you could lose characters off the end of the domain if there are leading spaces before the email address…
probably better to
E=LEFT(E)
right at the start before searching for ‘@’
then just
L = LEN(CLIP(E))
and later on where there is
SELF.Name = CLIP(LEFT(E[1:X]))
you would then just say
SELF.Name = CLIP(E[1:X])
hth and cheers for now
Geoff R