According to Ken Rosenblum, who originally noticed the problem, Apple's filtering of outgoing MobileMe messages counts as censorship.Apple May Be Invisibly Filtering Your Outgoing MobileMe Email [Updated With Apple's Response] (via Super Punch)
"If Apple or anyone else is going to block/filter/censor emails, they should at least notify the customer that the message did not go through," said Rosenblum.
While it's true that most hosts do some sort of content filtering with outgoing emails -- for example, for sending too many messages during too short a period of time, or autoresponding to too many people -- Apple's MobileMe filtering appears to be unique in that it is based upon the content.
Presumably, MobileMe's outgoing email filtering is a preventive measure to keep spam from being sent through Apple's webmail servers. However, since there's no information on what content triggers Apple's anti-spam conditions, and since the offending emails aren't returned to sender with an error message attached, Apple's MobileMe email filtering has all the superficial appearance of censorship.