Emial sendind limitaion in Microsoft Dynamics AX using SMTP mail server

When send email from AX one by one that working fine. But when email more than one in sequence that send only 45. e.g

When send email in bulk; like 500 email in sequence and loop running and send email in sequence without delay. system send only 45 and stop with out any error. kindly provide any solution for increase their limit from AX or SMTP server.??