Multiple messages in Mail

Hello,
This is my first post.
I have a big client database, and before I switched to Mac, I used Eudora as my mail program.
With Eudora, you could create a group, like "European clients" and in the subsequent window, type as many e-mail addresses as you want, let's say 300. Then you would click in the upper bar "send message to" and a pop up menu would come up with "European clients" being one of them. You could then send a mail to your 300 clients.
With mail, since it's related to your contacts, you need to create a contact first, and then create a group where you would drag all the contacts you want to be in this group.
The problem is that I have a list of 5000 clients and most of them are NOT in my contacts.
Anybody knows a trick, how to, or a software in order for me to create a group and just type or paste a bunch of e-mail addresses into that group?
Thank you very much...