Ok, thanks. If I understand you correctly you use a number of commercial email providers (iCloud & Google) to perform as "Spam" filters for inbound emails to your business email server. It doesn't matter if this server is hosted at your business or being provided by a third-party, in this case.
On the other hand, emails going outbound from your business server is being "blocked" by, at least, the iCloud server. Is this correct? In my experience, outbound emails that get blocked are due to an issue with the sending server ... in this case, that would be your business server. Typically, because the server is "blacklisted" by either: your ISP, the email server's ISP, or the email server itself.
I'm not sure if there is anything I can do to assist you with this as I would be looking at your email server as the place to start troubleshooting. Good luck!