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Do you mean text messages from this specific person are actually being emailed to you? That is, you have to open Mail.app to see the messages? Or are you saying that the messages you're receiving from this person are still going through Messages.app but are coming from an email address?
 
Settings > Messages > Send & Receive > Start New Conversations From…

They probably have their email address configured as their default sender address. Nothing to worry about.
 
No, i mean imessage on their end is sending using their email rather then phone number

Either that or they're actually texting your email address. My parents do it all the time accidentally and it gets super annoying. I have to go into their phone and delete the thread that's sending to my email and make sure the one sending to my phone number is the only one there.
 
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