I can give you some tentative answers, but for an official response you'll have to wait for SmarterTools...
1. Mailbox Data Encryption:
SmarterMail doesn't have any built-in encryption tools. You'll have to rely on third-party tools (e.g., BitLocker).
2. Audit Logging:
Yes, you can find it in the Administrative LOGS and in the LOGS of the various protocols (POP3, IMAP, SMTP, MAPI, EWS, EAS, etc.)
If yes, what level of granularity is available (e.g., login attempts, configuration changes, message access, etc.)?
It's quite granular, and you can set the log levels between Exceptions Only, Normal, and Detailed.
However, it's up to you to decide if they're complete enough; there's no exhaustive documentation.
In my opinion, most of the logs are complete enough, but others (especially the ability to see user activity such as deleting/moving/etc. on emails) are a bit less comprehensive than I'd like and are confusing (compared to other products like Kerio Connect or MailEnable).
3. Authentication:
SmarterMail has limited integration with LDAP or AD, but I don't believe RADIUS.
4. Session Management:
Is there any enforced limit on concurrent user or administrator sessions? NO
Is it possible to define session timeout durations for security purposes? YES for webaccess, NO for others
Gabriele Maoret - Head of SysAdmins and CISO 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)