If you have installed the first publicly available version of AADSync (v1.0.0419.0911), the eventual successor to DirSync, you might have noticed that the ProxyAddresses attribute will not get synchronized to Office 365. Unfortunately this means that all proxy addresses will be gone in Exchange Online!
It turns out this is not a configuration error, but a bug with this release... Microsoft statement is that “currently Proxyaddresses will not work with AADSYNC, and will be addressed in the next release”.
As such, you have two alternatives:
1. Update to the latest version of AADSync, v1.0.0470.1023 (obviously recommended!);
2. If, for some reason, you don’t want to upgrade, edit the existing rule to sync the proxyaddresses attribute.