Unfortunately, as originally reported by John Brownlee at Cult of Mac, it looks like Apple may be filtering out emails sent from the online MobileMe client based on content. The story is that a user was trying to send an email mentioning "Authoritarian Oppressive Regimes," and when sent, those messages would not go through to their intended recipients, with no error message back from Apple. Cult of Mac sent a few more test messages through the system, and found that simpler emails with less political messages made it through just fine.
There's a few things to note here: First, this is the web client only -- there's no indication that any emails sent via MobileMe's SMTP/IMAP interface are being blocked in this way. Second, Apple has replied to the CoM story to say that certain messages caught by spam filters may be blocked from sending through the web client, but that "Apple is not blocking MobileMe email due to political content." Anyone having a problem sending anything through MobileMe is encouraged to contact MobileMe support.
Perhaps most importantly, when Apple's rep was asked why no error message was given, there was no further comment. That's perhaps the biggest problem here -- Apple can do what it wants with its own service (even if that's to block messages based on political content, as unlikely as that actually is, a private service can do it), but Apple should at least notify users when messages are being blocked.
You could argue that Apple is trying to discourage mass email spammers from abusing the MobileMe system, and because of that doesn't want to clue spammers in on which message can and can't go through. That's a legitimate concern, but even so, if non-spammer customers are being blocked, there needs to be some way for them to be informed about the issue so it can be fixed.