I have spent the last couple of days trying to figure this out. I'm sending reports to our CEO (telco) through SQL Server Reporting Services. The emails are in MHTML and when they exceed a certain size limit, the Mail app skips all pics (which look like attachments).
Since there is absolutely no documentation on the Mail app and no way to change its settings or debug it, I started trying to test with different email/attachment sizes. So far I've figured that all emails under ~140-150kb get downloaded in full. When I exceed the 140kb threshold, I start getting issues. Still, it seems like it's not a simple limit which governs what gets downloaded. When my email is heavy on the attachments, or contains only attachments, everything under 140ish kb seems to work fine. However, if I have some text, things get messier. I have a report which Outlook tells me is around 235kb and it renders ok, but when it gets to like 240kb, it doesn't.
All this leads me to think that Apple really should disclose any thresholds which are hardcoded in the Mail app (and yes, it's not an ActiveSync mailbox policy either since that's unlimited on our Exchange 2007 servers) if it wants to present its OS as a robust and enterprise-ready platform, which it currently is not. Even better, why not expose these so we can set them somehow? And why not rely on ActiveSync policies for these so we can manage this properly?
For now, our execs will be using PDFs until we get more info from Apple.
And no, it's not possible to drop SSL (how does this matter anyway?), or to change the sender. It's plain HTML, it renders fine under certain size and there is absolutely no excuse for not letting us know how this works.