Fix Linux Centos 7/ Plesk email delivery -- 2

We have migrated a server onsite our emails deliver to our exchange server and everything works fine with this but our websites are hosted on a linux centos 7 running plesk obsidian. When processing webforms or order emails they are not delivered to our exchange server they sit in a queue whether it is postfix or qmail neither will send. Tried to use SMTP provider on websites using a number of different providers test emails will send but forms etc will not. Need to configure the server to send correctly.