According to Mac OS X Hints, if you're trying to send an email to someone with a spamcop.net email address with your MobileMe account, chances are the message will never arrive, and you won't be notified.
What's worse, apparently if you're sending the message to a distribution list, and only one of those recipients has an address at spamcop.net, nobody will receive the message.
Apparently the problem has been happening for months, since Apple moved everyone from .Mac to MobileMe. A participant in the Apple Discussions thread notes that it's common for service providers to filter their outgoing mail by domain in order to avoid being blacklisted. So far, only spamcop.net appears to be affected.
A workaround for now would be to try to send the message using a different email account. We'll let you know if Apple offers a fix in the meantime.
Update: Friendly reader Jason sent me a detailed explanation about why this might be. In a misguided attempt to control their individual load of spam, some users choose to forward all their MobileMe mail to a spamcop.net address. Spamcop, unfortunately, thinks the "spam" originated at MobileMe, not the actual origin of the spammy badness. So, MobileMe, to combat this, forbids forwarding to domains like Spamcop in order to avoid being blacklisted. There's nothing sinister going on here, just honest network administrators doing what they can with what they've got. Thanks, Jason!