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.