Scam emails

Yesterday I received an email from a friend showing their correct email address. It asked if we could email but said they couldn’t talk. I replied that I had no problem with that and sent a bit of chat back. They responded that they were pleased I could help so I wrote back that they hadn’t told me what they wanted help with. This was the response:
“Happy to hear from you. I need to get an Apple eGift Card sent to my niece, It's her one year of beating cancer, and I've tried to order it online but my card was declined. I promised her as a gift I'm currently in the hospital because of my throat pain caused by laryngitis. Could you help me order it online from Amazon or Apple through her email? I'll reimburse you once I get back home. Hope to hear from you soon. Thanks Tony”
Immediately I recognised a scam and sent back ‘NO!’ and reported it to Action Fraud.
My question is: how can a scammer send and receive messages on someone else’s email address?
I’m now going to be distrustful of every email I receive since I can’t be sure who’s sending it.
Has anyone else experienced this and is there a way to easily identify the email isn’t genuine?