classic problem with emailing groups on MS products
you would imagine that by now the interface/training would have changed to stop this.
With exchange someone knocks up a group for convience and as it shows a single entry on the "to" field so they go ahead like they would for internal message but since it is going out vias SMTP the email has each recipient rather than just the group name on the "to" line.
MS fail due to showing their own dialog rather than a specific one for SMTP where the user could see who is getting what.
Even a question box saying one of more recipients is going to have the full mailing list are you sure? would be enough