Vista - Number of recipients or message quota reached

Looks like I've sent too many emails:
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The message could not be sent because its size exceeded the server's limit.
You can use the option, located in Tools | Accounts | Properties | Advanced,
to break messages into smaller parts. Subject 'Re: Gorge Udate, Account:
'Live OE', Server: 'smtp.live.com', Protocol: SMTP, Server Response: '552
5.3.4 Requested action aborted; number of recipients or message quota
reached', Port: 25, Secure(SSL): Yes, Server Error: 552, Error Number:
0x800CCC6D
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ISP is MSN via smtp.live.com
They were html emails, no graphics and a short list of announcements an
events.
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It's the first time I've used a bulk mailer (GroupMail) and also the first
time I've tried to email 400 association members. 175 emails were sent
successfully, but the remaining emails were blocked--

--and I'm still blocked from sending "any" emails, even a single email

What is the number of recipents or number of message I can send at one time?
How long will I be blocked from sending more messages?
Is there a suggested setting for sending this number of emails?
Anyway to reset and undo the block?
Each message is to an unique email address and there are no CC or BCCs, etc.

How should I schedule these mailings so I don't land up getting blocked --
or where can I find this information?

Many thanks!

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Pat