Apparently there's a "best practice" that needs to be re-thought at this one company. The idea that the Primary Domain Controller (PDC) should serve several roles rather than simply being the backbone it is supposed to be. I mean, after all, there are Backup Domain Controllers all over the place, the PDC isn't that important, it can do more.
Well, This PDC was the main LDAP store, it also seems to have been the key mail server. I'm not sure what other roles it served, but this was apparently enough. The problem came when the DC freaked out, causing mail to stop and hundreds of e-mails went into oblivion.
Dedicate a few servers... Don't virtualize everything, and use redundancy.