To me, that sounds like a configuration of the server. SMTP servers commonly define a maximum amount of users that an email can be sent to (designed to prevent extreme load on the servers, and avoid being used for spam).
If you could provide more detailed information about your problem, such as how you're addressing the emails (address book individuals, address book groups, manually, etc.), and how many people you're trying to send emails to when you get these messages, I'll try to give some more help.
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