We recently migrated from Google Apps Sync email to Exchange 2010.
We are having the same problem as posted here with NDRs when replying to emails pre-migration:
No resolution or even an explanation there though.
Basically, replies through Exchange to pre-migration emails (internal and external) received by the Google Apps will bounce back from the local Exchange with a 5.1.1 NDR like this one:
Diagnostic information for administrators:
Generating server: SERVER.xyz.local
IMCEAEX-E@xyz.com#550 5.1.1 RESOLVER.ADR.ExRecipNotFound; not found ##
The letter suffix after "IMCEAEX" varies depending on the intended recipient. Replies to emails received both from internal and external recipients after the migration work fine, even though to all appearances the SMTP addresses are exactly the same.
The migration was done by importing the Google Apps .pst into the user's Exchange mailbox. Google sync is not loaded in the user's profile.
I can provide a bit of other diagnostic info:
It is not a problem with the Outlook autocompletion (don't I wish). Both the cache and the stream_autocomplete files and suggested contacts have been emptied several times with no effect.
It also doesn't seem to be the usual issue with the LegacyDN or X500 addresses, since there was no Exchange server prior to this. At any rate, that all looks OK from what I can see in ADSIedit and EMS. There also is nothing in the properties/headers of the message being replied to that indicates anything amiss.
I did try adding "IMCEAEX-E@xyx.com" as an SMTP alias for the internal user it was trying to send to. Exchange still bounced it both from inside and outside. (Interestingly "IMCEAEX.E@xyx.com" works fine.) So Exchange seems to be partly implicated in this, but I can't find anything.