Simple Mail does not support random selecting multiple email-addresses(usually via Ctrl- click) in "To", "Cc" or "Bcc" fields while compositing a message. But via defining "Group" in address book, users may add to those fields with fixed set ("group") of multiple recipients/email-addresses. You can create several groups to fit your needs.

How to:

1. Click on "Preferences" icon on tool bar to open and select "Address Book" tab for setting.
2. Click on "Add" to bring up the dialog, which can be resized to your likes.
3. Give it a name in "Name" field and check the "Group" option for defining a new group.
4. Keep on double-clicking contacts in address book for adding members to the group until it's done.
5. Click on "OK" and there you are.