It looks like we're discussing two types of mail here: Fan Mail and Email.
Fan Mail is a "HubPages thing." When you follow a hubber, there's a window that pops up allowing you to send that hubber one piece of fan mail, which is visible for all of Hubland to see. We can only send one fan mail to each hubber. Just one ever.
If you didn't write a fan mail when you first followed someone, you can still go back and do it from their profile, like Alphadogg was saying. There's "Activity...Content...Followers...Following...FAN MAIL" Click it and you can send them a fan mail.
The thing is, when you send somebody a fan mail, they can't respond. So when someone sends you a fan mail, you can't just hit, "Reply" like with a regular email.
Buuuut...you can just send them a regular email, like rebekaELLE explained. This is also accessed from the PROFILE page -> FAN MAIL page -> "send [hubber's name] an email"