So a weird week plus for me with Office 365 Exchange Online pilot projects. The great news is I am either the lead consultant or serving as the project manager for five Office 365 projects right now, demand and interest in Office 365 is very high. Now to the bad news, with two of these clients we ran into issues with mail flow between the on-premises hybrid server and FOPE/Exchange online. It took some digging on the first client that this came up on and finally we were able to trace it to the Router provided by the client’s ISP that had ESMTP packet filtering enabled. It took us some time on this first client as it was one of the last places we looked (duh Sean, why keep looking after you found the problem), because we could not fathom that a ISP Router would be filtering packets. So after getting the ISP to remote into the Router and turn off the filtering everything worked good.

Armed with this knowledge, the same issue came up at my client today. With the recent experience I was able to work with the network team to drive the troubleshooting and get this resolved rather quickly. So now I think for every Office 365/Exchange Online client engagement one of my first questions, right after asking about Public Folders, will be do they have ESMTP filtering enabled.