I have this same issue with 1 single account on a single onpremise server with local Outlook clients.
It's a small server (only 17 users on a single domain), 16 working like a charm, 1 started to have this issue a few days ago (previously it was working).
I have an open ticket, but no resolution...
Gabriele Maoret - Head of SysAdmins at SERSIS
Currently manages 6 SmarterMail installations (1 in the cloud for SERSIS which provides services to a few hundred third-party email domains + 5 on-premise for customers who prefer to have their mail server in-house)