Apple Support Article #TS2883:
MobileMe Mail has a message sending limit of 200 messages or 1000 recipients per rolling 24-hour period. For example, if you send 100 messages at 8 a.m. and then 100 messages at 12:00 noon, you would reach your message sending limit at noon, but you would be able to start sending messages again at 8 a.m. the next day.
I wasn’t aware MobileMe had any in place. It seems Gmail has them too:
In an effort to fight spam and prevent abuse, Google will temporarily disable your account if you send a message to more than 500 recipients or if you send a large number of undeliverable messages. If you use a POP or IMAP client (Microsoft Outlook or Apple Mail, e.g.), you may only send a message to 100 people at a time. Your account should be re-enabled within 24 hours.
MobileMe is a lot more forgiving, but I suppose Gmail — being free — is a much bigger magnet for spammers.