Sometimes when you launch Office 365 Hybrid integration wizard from Exchange 2010, after successfully implementing the integration the IPs of Microsoft are not all imported in the "Receive Connector" for Microsoft so you might have to consider to add them manually to your on-premises Exchange server.
In order to do so, Open Microsoft Exchange Management shell as Administrator and follow the following cmdlets.
[PS] C:\>$RecvConn = Get-ReceiveConnector "Inbound from Office 365"
[PS] C:\>$RecvConn.RemoteIPRanges += "65.52.148.27", "65.52.184.75", "65.52.208.73", "65.52.240.233", "65.54.80.0/20", "65.54.165.0/25", "65.55.86.0/23", "65.55.233.0/27", "70.37.128.0/23", "65.54.54.32/27", "65.54.55.201", "65.54.74.0/23", "70.37.142.0/23", "70.37.159.0/24", "94.245.68.0/22", "65.55.239.168", "70.37.97.234", "94.245.86.0/24", "94.245.117.53", "94.245.108.85", "94.245.82.0/23", "94.245.84.0/24", "132.245.0.0/16", "157.56.23.32/27", "157.56.53.128/25", "157.55.155.0/25", "157.56.55.0/25", "157.56.58.0/25", "157.55.59.128/25", "157.55.145.0/25", "157.55.185.100", "157.55.194.46", "157.55.227.192/26", "157.56.151.0/25", "157.56.200.0/23", "157.56.236.0/22", "207.46.216.54", "207.46.57.128/25", "207.46.70.0/24", "207.46.73.250", "207.46.150.128/25", "207.46.198.0/25", "207.46.206.0/23", "213.199.148.0/23", "213.199.182.128/25"
[PS] C:\>Set-ReceiveConnector "Inbound from Office 365" -RemoteIPRanges $RecvConn.RemoteIPRanges
Hit Enter after each PS line and you will be able to find all those IPs in your connector.