We use Exchange 2003 and Trend Micro IMSS 7.x, sometimes messages are bouncing back to the sender. Please refer to the email below which I sent from my Gmail account to my company account and it bounces back.
Thanks and regards,
Diagnostic code: smtp;552 ekfcegw.emiratesabela.co.ae: your message counts in one session of 504 exceeds maximum setting 500
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