I would have guessed that Frank's suggestion would have worked for you.
And Andy's suggestion should work, too:
=HYPERLINK("mailto:x@x.x","(e-mail address removed)")
But I'm wondering if simply typing:
mailto:x@x.x
Accomplishes the same thing.
And what does it didn't work mean?
Did Frank's suggestion insert the hyperlink, but it didn't start OE?
Or it didn't get converted to a hyperlink (clicking on it didn't do anything)?
Or it got converted to a hyperlink, but it didn't look like one?