We have updated to 15.5. Same issue. Today 2 users, one with outlook 2013 and one with Outlook 2016, lost some emails. Logging in to webmail everything is ok. We are getting problems with active sync almost everyday and we are very disappointed because this problem is not new to smartertools but seem that nobody cares about this. The offical answers are also the same: "Update to latest version", or "buy a ticket" or "we don't have any problem in our tests".
We are workarounding the problem using IMAP on pc and using activesync only on mobile phone.