I need to underline part of the e-mail address and I don't have an
underline icon that's available.
What version of Outlook? Sounds like you're composing in plain text and
you need to compose in html or rtf. Unless, of course, you're trying to
underline in the To field, in which case, you can't, which makes me want
to ask why you need to do it. Well, the Subject line would be out too, no
formatting is allowed there.