Using Connection Filter in Office 365/Hosted Exchange in conjunction with AppRiver

Hello experts.

We use hosted exchange in office 365 in conjunction with AppRiver for spam protection.  All incoming emails are supposed to go through appriver before being approved and then forwarded to office365/exchange.  

What has been occurring is that some spammers are sending directly to office365 thereby bypassing our spam protection altogether.  I looked at some recommendations from AppRiver and in theory all I need to do is add a list of their IP Addresses and Networks to the 'IP Allow List' under Exchange--Protection--Connection Filter, and then block everything else (if email is received from an IP on both lists, it will be allowed).  

"IP addresses must be specified in the format nnn.nnn.nnn.nnn where nnn is a number from 0 to 255. You can also specify Classless Inter-Domain Routing (CIDR) ranges in the format nnn.nnn.nnn.nnn/rr where rr is a number from 24 to 32. IPv6 address can be input but messages sent from these addresses will not be accepted by the service. "

Because of the CIDR Range limits listed above, I am unsure how to go about blocking large ranges of addresses, or if I am even aproaching this problem in the right way(I have only been working with Office365 for about 4 weeks).

Thanks in advance.
ortherAsked:
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Adam BrownSr Solutions ArchitectCommented:
What you need to do is modify your default Receive connector so that it will only accept email from App-River. Since you're on office 365, you can do this by going to Exchange->Mail Flow -> Connectors then you can create a new Receive connector. Click the + sign, then Fill out the wizard that pops up. Enter * for the Sender Domains then enter the IP Address range for App-River (should be able to get this from their website somewhere) under Sender IP Addresses. Give the connector a name and leave everything else default. This will make sure that the only IP addresses that are able to send to your tenant are from App-River, so you won't have to have a huge list of blocked IPs, just a list of allowed ones.
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ortherAuthor Commented:
"The Hybrid Mail Flow inbound connector was created when hybrid mail flow was configured.  This connector cannot be modified."

This is in the comment box of the only inbound connector I see currently.  Will that be a problem?
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Adam BrownSr Solutions ArchitectCommented:
No. Just add another connector and set it up accordingly, but make sure your On-prem IPs are in Sender IPs as well.
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