Here's some detail on what went on... One of the things last week's release was supposed to include were some memory optimizations to our TCP code which is used by POP, XMPP, SMTP and IMAP. (Not EAS, EWS or WebDAV). These optimizations were to help streamline processing requests for those protocols as well as address a memory leak that was found. Unfortunately, what appears to have happened is that the code also caused commands to be half-read or for some requests from clients to contain incomplete commands. In either case, clients would drop connections altogether or error out with incomplete info written to the logs. The devs stepped through all that code again and were able to narrow down the issue and get it resolved. We were able to keep the optimizations while mitigating the odd behaviors in SmarterMail 16.3.6821 and 15.7.6821.
Obviously, we're never pleased when bugs like this get released. We do have manual QC processes that occur before any minor goes out. 99% of the time, we find issues prior to release. Some issues are harder than others to see ahead of time, and, of course, everyone's environment is different with different protocols being used, different user counts, different clients, etc. We try to be proactive with testing and QC, but each install is different and user mileage varies.
That said, Tim is obviously involved and we are actively discussing how to catch these types of issues more efficiently.