We create emails that look beautiful on Macs (all clients) and terrible on Outlook for PCs. The HTML version always looks perfect, of course, no matter the client.
It's understood Outlook is the hardest to code for because [a] its HTML rendering engine is the most limited overall (and that's in a field where HTML feature support is already back in the HTML4 days), [b] it has more non-standard features (like VML), and [c] there isn't just one "Outlook."
That's what Litmus/Email on Acid/a dedicated + experienced email designer is for.
Hi Andrew - Yes, Outlook seems to cause the most issues when coding emails or email templates. I would recommend creating a master email template with all of the modules you will need to utilize for all types of emails. You can ensure this passes in all clients by testing with email on acid or litmus. Once you have your master template, you should save a ton of time on email creation moving forward.