some of the mails are not delivered

Hi,

we are having O365 and customer having Exchange server 2013, whenever we are sending mail some of the mails are delivered and some of the mails are not delivered. they are having TrendMicro IMSVA Scanner, they logged case with TrendMicro and they are stating some of the mails are not hitting the Trendmicro IMSVA. Please suggest where to troubleshoot.

Regards,

Mani