Hello there Gabriele! Is it possible this user has more than just the MAPI account set up in Outlook? I don't think it should be possible to have mail in Outlook but not in webmail when using Outlook MAPI since it syncs the folder contents down from the server itself, and so I'm wondering if this missing mail might have come in through a different account?

On your question specifically; it depends on where this mail came from and what state that mail data is in. It may not be a bad idea to export the user's Outlook profile to a PST file so you have it available before you make changes. 

This user has only one account configured in Outlook (the one they normally use, a MAPI account).

Missing messages on the server (and present in Outlook) are messages SENT from March 3 to yesterday.

All other messages are OK, even messages sent today ...

I don't know why, but today it seems to be fine ... Could it be because I updated SmarterMail to the latest version last Saturday?

That's quite possible Gabriele. It sounds like their Outlook client got itself out of sync with the server and it resolved itself with a subsequent sync update. Keep an eye on it and let us know if you see it come back :)

Hi Kyle! Tomorrow I have a call with the customer to verify his Outlook.

I'll let you know.

Sounds good!