Probably the spamfilter is the cause of your trouble, but I can't duplicate the error on my blog. Try uploading the most recent filter from the backoffice -> Antispam.

Check your blacklist, it *might* have mac.com in it. If it does then just click the green tick and the entry will be removed ;)

¥

I checked the blacklist and indeed, mac.com was there (why didn't I think of that :) ). Clicked the green V and now the link is accepted. Tnx for all the usefull tips!