You are probably talking about received emails, not ones your are creating or forwarding, yes?
Because the displayed email isn't in an editable box selecting it is slightly different. Instead of tapping and holding as you would in an editor, double tap and hold on the second tap. This should select a paragraph or two, and then you can drag the handles to increase or decrease your selection.
In my experiments you do not have the fine control over selection that you do when editing. For instance you can't select words or sentences. Paragraphs or at the most lines are the the units you can deal with.
Still, when you paste it to you destination you can fine tune the resulting text there.
I've found that selecting stuff on a web page is erratic and unpredictable. Outside of an editable text field you never know if a single tap hold will work, or whether you need to double tap and hold. Also the way the selection handles work varies. It's probably a matter of whether you happen to tap on some HTML, CSS, or whatever code. It's almost impossible to tell by just looking at the page.
