Autocomplete only works if the entire string matches from the beginning to the point where you are
typing. If the ending is the same (the @etc part), you could use a column of formulas to add that
part to the beginning part of the address:
With
firstname.lastname
in cell A2, the formula
=A2 & "@etc"
will return
firstname.lastname@etc
And then copy that formula down to match your list.
