need MAIL command need RCPT command error when sending to many users

hello,

In our stand-alone Exchange 2007 server we have a problem as we send mail to many out-of-the-organization users. When number of users is high (cannot tell how many) email messages will not be delivered.

If I look at the message tracking I find several 503 5.5.1 Error need MAIL command or 503 5.5.1 need RCPT command (I think as much in each email message as recipients it has)

Our send connector sent email messages directly to a relay server our provider has. It Works fine in the rest of situations.

Any idea of what might be happening?

Thank you very much.

Best regards.

David