our current email infrastructure details are described below..
We have a on premise Linux based mail server with "@domain.com "
smtp address. and exchange on-line (office 365) users with same smtp address (simple domain sharing for SMTP address ) .
Mx record are pointed to our local email gateway and this mail gateway also mange mail flow . i.e.. one mail came from outside to our email gateway ,then email gateway are verifying the destination email address is on premises or office 365 . after verifying deliver to destination address
our problem is when sending to a mail from outside domain like gmail ,yahoo and etc.. to office 365 user mails are moved to junk folder ..what are the possible reason for this....