Several threads say the problem is the mail provider (gmail, Earthlink, etc) not Mail, which is technically unlimited. There is a lot of info in the Snow Leopard (your OS) forum- go there https://discussions.apple.com/community/mac_os/mac_os_x_v10.6_snow_leopard and choose Refine This List and select "Mail and Address Book" and do a search for max recipients or similar wording.
I gets lots of emails from groups that send emails with each individually addressed so no one's email address is published. Of course, all listserv's do this, but I'm not a server. On a PC, people can use Microsoft's database software for this, but there's other mailer software in use on PC's too. That's the sort of thing I'm looking for.