IDS Rules dont work as expected - uses get randomly blocked with no reason or log.
Search functions specific to logs are broken, either no results are found when searching for a specific ip, user etc.
unable to restore multiple emails from archive, interface does work - random errors thrown on second or third attempt to restore mails
random corruption on grp files
the performance is +- 200% slower on a good day
Mail migration doesnt work
buggy interface for domains list - searching for domains with specific key phrases doesnt work
Unrelated results in log search
IDS Rules not being removed
Buggy Archive Search
Unable to set postmaster return address
adding a new user via api changes the logged in domain admin's session and you need to re-impersonate the user.
Rather upgrade to 16 if you feel the need.
As an ISP we work on the mail servers on a daily basis, trying to assist clients with delivery issues etc, and if this is similar to what you do then wait until MAPI has been released, and Smartertools have caught up with all the outstanding issues with Smartermail.
Right now the latest version offers one bonus, and that is 2FA - if you're willing to live with a mail server that makes a mail administrators life hell then upgrade to the latest.