EOP to Exchange 2013 - mail flow delay

Hello all,

I've issue with mail flow dealy. I setted up EOP recently. So dns mx record is point to EOP and EOP forward emails to MS Exchange 2013 on premise. But there is mail flow delay for some emails. The delay occurs betwen EOP and our internal Exchange server.

When I check mail flow on EOP, there is information: Message sent to our.mail.server at 9:00UTC but email arrived at 9:21UTC. When I check email header I see 21 minutes of delay between EOP and our mail server. There is not antispam on our router.

Where should be an issue? What should I check?

Thank you.