Hello,
I'm one of Marketo's antispam experts. This isn't a behavior I'm familiar with causing problems, but the antispam world changes on a pretty frequent basis. I'd like to hear more about the specifics of your tests and bad results - does it look like this is specific to a particular ISP or antispam vendor? What specific testing system are you using?
Whenever you have a few, I'd love to hear more. If you'd rather not share here in public, please send email to my team's alias, privacy@marketo.com.
Thanks,
Autumn