same problem here
Updated to 2.1, read the new features threads but double tap zoom doesn't work in email. Only the reverse pinch / spread does the trick.
It is an odd one to imagine working however. Double tap zoom works by finding a division (column of text, table cell, etc) and zooming to its edges. In the case of these HTML emails they are displaying with infrequent linebreaks at very long linewidths, and text is too small to read. When I pinch zoom I enlarge enough to read the words and then manually drag back and forth trying to read the lines.
Does double tap zoom really pick an arbitrary zoom level, giving a close up on a partial line of text so that you can do the awful scrub left to right and back??
Why doesn't the mail app just reflow the lines?