A number of our users are complaining that HTML email from external senders are showing up as source code instead of displaying as intended (tested in both Outlook 2007 and OWA). I wanted to ask if anyone knows of EXCH 2010 settings that may affect the appearance of some html emails. If not, any idea what could cause this? THanks!

Usual cause is third party interference. AV, Antispam, firewall scanning SMTP traffic. Something breaks the email so that Outlook cannot render it. The fact that it shows the same in OWA is a clear indication that the email is damaged in some way. Check you have the correct exclusions in any AV on the Exchange server and any firewall isn't scanning SMTP traffic. Simon.Simon Butler, Exchange MVP Blog | Exchange Resources

Thanks, Sembee, I'll have them check that out!