There's definitely no hidden option linking Archive to Older in any of the clients. The Archive folder only appeared after installing Sparrow and was already set to its default Archiving folder. The other clients don't have the ability to set an archive folder assignment like their do for inbox, trash, draft, sent.

When I look at folders using iOS default Mail app that's where I can see that it has a special "archive" image instead of a generic folder image that any random folder would have.

I'm going to try accessing the server with SSH to look for a .subscription file and I think I'll also remove Sparrow from my phone and re-install it at a later time. That way I can take it out of the loop completely while I test.

And for Tony, nope, I don't have any other machines configured to access these accounts - only the iPhone and Mac.