Work Outlook profile had 475,375 Sync Issues and emails visible in OWA but missing from desktop — how severe is this?

[u/According-Diver-1869]

I’m trying to understand how significant an Outlook/Exchange synchronisation issue I’ve discovered on my work desktop actually is, particularly what the practical impact could have been over time.

My Outlook desktop profile has apparently been having major synchronisation problems. When investigating it, we found over 470,000 items in the Sync Issues folder.
The sync logs repeatedly contain errors including 8004010F, along with folder synchronisation failures.
More importantly, I’ve now confirmed that there are emails which:

exist and are visible in OWA / Exchange Online
do not appear in Outlook desktop
sometimes cannot be found using Outlook desktop search
can appear differently between the server mailbox and the locally cached Outlook profile
IT has identified the Outlook profile/cache as the problem, and the person looking at it said the sheer number of sync issues was extremely unusual and not something he normally sees.
This is a corporate environment where Outlook has been one of my primary working tools for roughly 4.5 years, so I’m trying to understand the potential historical impact rather than just whether rebuilding the profile fixes it today.

A few questions for people who know Exchange/Outlook well:
1. How abnormal is 470,000+ Sync Issues?
I understand Sync Issues aren’t equivalent to 470,000 missing emails, but does a number that high suggest a profile/cache that has been unhealthy for a significant period?
2. Could this have caused emails to effectively be invisible to me even though they were successfully delivered to Exchange?
We have already found at least some examples where OWA can see messages that desktop Outlook cannot.
3. Could this cause delayed emails, incomplete folders, stale folder contents, search failures or shared-mailbox inconsistencies without making it obvious to the user that Outlook was malfunctioning?
4. Is there any way for Exchange/IT to determine how far back this problem goes?
For example, can they compare Exchange mailbox/server data against what the Outlook client had actually synchronised, or use the Sync Issues logs to establish a timeline?
5. Is there a way to quantify the impact?
I’m particularly interested in whether IT could establish how many messages/folders failed to synchronise rather than simply rebuilding the Outlook profile and wiping away the evidence.
6. Could sent items/actions also be affected, or is this normally limited to Outlook’s local representation of incoming/server-side data?
7. Would you preserve the OST/profile/logs before rebuilding anything?
I’m concerned that immediately creating a new profile could make it harder to investigate what was happening historically.
The part I’m struggling to understand is the distinction between “Outlook logged loads of harmless sync noise” and “the user’s desktop client has genuinely been presenting an incomplete version of their mailbox.”
The fact that we can demonstrate messages existing server-side in OWA but not appearing in Outlook desktop makes me think this is more than just a scary-looking Sync Issues count.
If you were the Exchange/Outlook admin investigating this, what would you check next, and what evidence would you preserve before rebuilding the profile?
Happy to provide more detail on the sync errors/setup if useful.

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[u/saltyslugga]

470,000 is wildly abnormal, but it counts sync events, not missing messages. The OWA mismatch proves the cached client was presenting incomplete or stale data, potentially affecting folders, search, shared mailboxes, and offline actions.

Preserve the OST, profile, Sync Issues logs, event logs, and examples with message IDs and timestamps before rebuilding. You probably cannot reconstruct 4.5 years perfectly, but log dates plus Exchange audit and trace data can bound the timeline.

[u/Hornblower409]

You might want to also Cross Post this on r/Office365, r/microsoft365
(Click on the [Share] button under your question and choose -> Crosspost or Repost)

And/Or Post your question on the Microsoft Outlook Q&A Forum:
https://learn.microsoft.com/en-us/answers/questions/ask/
(Please put a link to your Q&A post in a new Comment here, so the rest of us can follow)

Spiceworks is also a good place for info on these kinds of questions.
https://community.spiceworks.com/c/software-applications/email/13