Can I gather the associated email address from an iMessage that was sent from a phone number?
I see the mostly likely cause of this being the possibility that your default sending address is set to your email address. I say this because it appears that you initiated the conversation.

You can check (and change) this by going to Settings.app -> Messages -> Send &amp; Receive. Under "Start new conversations from:", observe whether your email address or phone number is selected.

If your email address is selected, this would be an easy explanation as to why this happened.

Otherwise, as far as I know, (and I have just went over this to verify), it is impossible to extract the email address that is associated with an iMessage phone number from just a regular iMessage "text".