Believe it or not, we are about 98% capable of making this happen!
The protocols were never intended to do this and Microsoft's approach has always been to delete and re-add accounts whether there was a server or client issue. They did try adding things like "Sync Folder" to accommodate some issues they have, for example it can recover missing items (sometimes), but that doesn't work for bad data.
We have been trying to work with Microsoft on a way to accomplish this for years and they believed it wasn't possible. However, over the last couple years we have moved beyond what the Microsoft protocol teams know.
Our solution will allow you to go to a user in a domain and toggle them to re-sync via webmail.
This will re-sync all protocols. This should fix probably 8 out of 10 issues. If a client is really screwed up, like it got disconnected so bad or local OST or PST's got corrupted, there is no way to recover from that.
As you can appreciate, this requires a crazy amount of testing and it's actively in QC and we hope to release it shortly.
Oh, another thing that it won't help with is when companies who have clients connected to Exchange and then move to SmarterMail but don't have their clients re-connect to SmarterMail. Even though SmarterMail is Exchange, Exchange holds certain keys and data that SmarterMail will never know about because it's not sent via the protocols. We have seen 1000's of these over the last month by a variety of companies transitioning to SmarterMail. That is a non recoverable situation.
The same would be true if a company went from SmarterMail to Exchange: The Outlook client would need to be delete and re-added to the new Exchange server even if it's using all the data server addresses, etc.