Maybe I'm being too simplistic, but doesn't your mail server allow you to create groups? i.e. Outlook: File>New>Group
For MAC Mail it says:
If you create groups in the Contacts app—say for a book club or cycling team—you can send messages to everybody in the group at once.
Type the name of a group in the address field of your message.
Either the group or individual contacts are shown. If the group is shown but you want to see individual contacts instead, click the arrow next to the group name, then choose Expand Group.
I checked out Contacts on my Mac and it's really, really easy to create a group.