Depending on your email client, you might just cut'n'paste it into a MIME/HTML capable mail client, and then highlight the word in there and click on the italics button. Of course, a lot of people, myself included, dislike recieving HTML mail. There are various modules that you might use to do this programmatically. One way might be to just set the mime type to text/html using the provided APIs, enclose the message text in a <PRE> tag, and then enclose the needed word in <I>WORD<\I>

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You might find these particular modules helpful:
MIME::Body
MIME::Light
MIME::Entity

MIME::Light will probably be the most useful.