Server running very slowly when mass mailing with postfix

hi all,

i've got a question about postfix.
I've got a Fedora Core 2 Server, and all is running well on it.

But, when i do a mass mailing with Ms Outlook to my customers (this NOT
spamming), my server run very slowly for a while.

To send mail, i put about 2.000 mails address in BCc.

Is the server slow because of 2.000 is too much for my server,
or because of it's the fact of putting them in Bcc ?

would the server be faster if it was 2000 single mail instead of
1 mail with 2000 addresses ?


I'm really sorry for my bad english


Fred