Neo,
I am using the Attribute Editor tab in ADUC to set the proxyAddress field. I believe that is the same as using ADSIEdit, correct?
If I modify other attributes of this user, like Department, the changes are successfully synced to O365. In fact, I tried adding a fake address to the proxyAddress field and it was also synced to the cloud. Then I removed the fake address and it was removed from the cloud. However, this one address that I'm trying to remove is not being removed. I believe it could be because the address I'm trying to remove was at one time the primary email address for this user. The old address still shows up in the "User Name" field in O365 even though I have changed the user name and email address in AD to something else. So maybe the problem is not with the proxyAddress field, rather the "User Name" field.
There are no errors in the event log for the last 48 hours that match that criteria. However, some of those sources I don't have. I am missing "ADSync" and "DirectorySyncClientCmd"
Thanks
Kent