How on earth do you put an underscore in an e-mail address directly in the "To" box?

Hi. My mother just asked me a tech question that I thought I could simply answer: How do you put an underscore in an e-mail address directly in the "To" box? When I say "To", I simply mean the input box that you enter the e-mail address to whom you are sending the e-mail. Mom got an e-mail address to send an important document, but this e-mail has an underscore, i.e. a line underneath a letter. Now we typed the full e-mail address into the "To" box. Then I tried to highlight that letter with the underscore and then clicked on the Underline button to underline that letter as the letter for the underscore. But it didn't work and the e-mail ended up being finished typed without the underscore instead. And that was when my mother was in Hotmail. Can anyone actually tell me what an underscore is really supposed to be as concerned to typing e-mail addresses directly in the "To" box. And can you tell me how to actually type in the e-mail address with the underscore included? Please I need someone's expertise on this because she needs to send e-mail to these addresses with underscores. I'd really appreciate it if you tell me how to type in e-mail addresses with underscores in Hotmail, because this is very confusing. R.S.V.P.