How can I copy an email address from a text message?
Revised for clarification. Use IOS's copy functionality to copy an email message. Press on the desired message (with email) and hold for a few seconds to bring up the Copy dialog. Click on the Copy dialog to copy the message (including email address). The entire message is copied. At that point, paste the message for further modification if desired (if it contains more than the email address).

Note: This worked for me in iOS 5.