At that scale, Synology NAS running MailPlus can be very cost-effective. Comes with five mailboxes. Buy two, activate MailPlus High Availablity — which also poo

At that scale, Synology NAS running MailPlus can be very cost-effective. Comes with five mailboxes. Buy two, activate MailPlus High Availablity — which also pools the mailbox licenses, and you're set for 10 mailboxes over the +/- 8 years they'll actively support the device. With the rest of their software suite it can tick all the boxes tho the lack of a MAPI/EAS implementation is a miss.