Mails From Exchange 2013 are received as SPAM

Hello,

We have Exchange 2013 configured to send and receive mails internally and externally. Everything is going great except the fact that the mails sent FROM our Exchange 2013 are received as SPAM. For example, I send an email from exchange to my gmail account (generally to any EXTERNAL receipt). I receive the mail in the SPAM folder.

Any ideas Please !!