The setup of the PCs is Ok. I can confirm that because I tested with other accounts and all work fine.
The size of the two mailboxes with issue is approximately 15GB for one and 2GB for the second.
I have bigger mailboxes (many are over 40GB) and no issue at all with them (tested on the same PCs...).
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)