I would love to see some sort of industry proof here; Title Case is pretty standard for email subject lines... Again, industry proof, not some guy working first level helpdesk...
MailChimp, for example, sends all of their emails as Title Case and I've yet to hear about their mail blasts being flagged by mail filters for using Title Case in them.
Bottom line, modify your forums to work how you see fit; unless I see some RFC's or guidelines by big names in the business I'm not going to recommend anything but Title Case.
In any case all of this seems speculative; any bounce backs should have a denial reason in the email headers; these should point to the precise cause of any email bounce-backs and what causes them.
