A brief hiatus hit Apple's .Mac Mail services yesterday, a note on the relevant support forums confirm.
It appears that .Mac members mail experienced problems between12:30 and 16:00 PST (20.30-00:00 GMT), though these problems have now been resolved.
"We are aware of an issue that may have affected the delivery of messages sent to some .Mac Mail addresses. This has been resolved. Messages currently being sent to .Mac recipients are being delivered as expected. If you suspect that a message sent to you has not arrived, you may wish to have the originator resend it," an Apple moderator warns on the support site.