Apple has been silently deleting email sent to Spamcop.net accounts, possibly it's a bug, possibly for fear that bot-captured MobileMe generated spam will add MobileMe to a blacklist ...
macosxhints.com - A caution on MobileMe outbound email domain filteringIt's the silent policy that's unforgivable. If this is policy rather than bug then Apple has jumped the shark.
This is a hint regarding a serious problem for MobileMe users: Outgoing mail via smtp.me.com may not actually be sent if it is sent to certain domains. If a message is caught in this domain filter, it is silently dropped without notification to the user. None of the recipients of the message will receive it, even if they are not in the filtered domain. This problem occurs with mail sent through Apple's Mail app, on the iPhone or on Mac computers. It does not happen when sending via the MobileMe webmail interface. So far, only spamcop.net has been identified as a domain filtered by smtp.me.com, but there may be others.
This problem is testable and repeatable as of this writing; you can test it yourself as follows. On a Mac or iPhone, use Mail to create a new message using your MobileMe account. In the To field, put firstname.lastname@example.org, in the CC field, enter a valid personal email address, and in the Subject field, put test, then send the message. You will not receive the cc message, and you will not receive an acknowledgment from spamcop.net. Try sending from a non MobileMe account; you will receive both very quickly.
More discussion of the problem can be found in this thread on Apple's discussions site. This has been happening for months, apparently since the transition from mac.com. The problem has been reported to Apple, and apparently some in frontline support are aware of it, but others are not. In the meantime, all MobileMe users should be aware that their outgoing mail is apparently filtered by domain. If you're sending to a spamcop.net address, be aware that the mail will not be sent to any recipient, nor will you be notified of the problem.
So that brother who's not speaking to you any more? Maybe it's because you didn't respond to the desperate email he sent from his MobileMe account a month ago, the one Apple silently deleted.