Hoople - 2:08 am on Jul 2, 2013 (gmt 0)
All of the above is great advice. Do check out that link 'limbo' posted, well worth your time!
I have seen recent changes in some webmail portal that affects css that may produce some unanticipated results. Example: Yahoo! email now seems to be ignoring floats but is honoring clears.
Just a reminder to test, test and retest BEFORE sending out your campaign. You may find some valuable lat minute tweeks!