It sounds like they were doing a 'mail merge' with a program like MS Outlook (not Outlook Express) and MS Word. Sure it works, but it's not very intuitive to set up a mail merge, and most ISP's are getting very restrictive with the number of recipients you can have in a message before your email is considered Spam and you get blocked. 250 recipients is probably on the high side of what an ISP would allow, but they are all different, and can change at any time.
You'll need to decide if you want to work with a Mail Merge and a different email program, or work with Apple Mail and your Address Book.
iWork 09 with Numbers and Pages is supposed to work with mail merge, but I haven't tried it. You convert the Excel file into a Numbers file for Pages to be able to use the data.
You can also use an excel spreadsheet as a 'data source' for a mail merge in MS Word to create emails for Entourage to send:
http://www.microsoft.com/mac/help.mspx?clr=99-4-0&srcid=875f9813-ee15-41b2-8603- 04d42c6b61671033&ep=9&target=d1b92bb2-9e90-4a90-9278-4a3fd0df57781033&CTT=Relate dLink
I haven't done a Word mail merge in years, since I got my Address Book set up properly.
There are Mailer programs that might do the work for you. Here's one:
1st Mac Mailer for Leopard -
http://www.download3k.com/Internet/Email/Download-1st-Mac-Mailer-for-Leopard.htm l
Related Thread:
http://discussions.apple.com/thread.jspa?threadID=1950019&tstart=29
Tutorials for different Mail Merge methods:
http://www.wonderhowto.com/how-to-mail-merge-pages-numbers/