Sending email from the wrong address?

Okay, this is a weird one.

I sent an email using the email application from my @sky.com account to a company regarding a purchase, however i didn't recieve a reply and thought nothing of it. It wasn't until later when I logged into my @hotmail.co.uk account to find out that my iPad had sent the message using that email and that i had recieved multiple reply emails. What i don't understand is that i have never used this Hotmail account on my iPad, and it's not even associated with my Sky one. Can anyone shed some light on the situation?

Many Thanks