Because of the issues reported on this forum, I'm sticking hard with SM V15.7.6970. This release is as rock-solid as can be. The issues I have now with SM are extremely minor and usually because of user error or ignorance.* We have a split-domain with SM + Office 365 and it's working out well.** Most users are still using SM.
I don't allow anyone to run Outlook on their desktops. On mobile devices it's fine, however. Interestingly, Outlook for Android supports ST's version of EAS as well as IMAP, while Outlook for IOS only supports IMAP, FAICT.
As for Server 2012 R2, you can get a new OEM version on ebay for $55.00 or less(!), and again FAICT these are legit. I have purchased these in the past. My rule in dealing with ebay is to ALWAYS read the listing, and in this case, make sure it says the product is NEW and in a sealed package. And pay with a credit card, whether or not you use PayPal. This gives you additional leverage if you have problems. Look for listings that show the white cardboard envelope with red tape across the seal, and "Microsoft Corporation" at top left. These do NOT include CALs, nor are they required for a Web/email server, per Microsoft licensing terms. And make sure that they are actually sending you this envelope, and not an emailed activation key.
To be fair, email server + webmail software with this many features is bound to be quite a complex product. ST is clearly working very hard to make the latest release of SM work well. Nonetheless, I'm going to stay with what works for me and my clients day in and day out, for the time being.
*Last night a user moved tens of GBytes of email around, and exceeded their allotted limit, taxing the server and making their email access inaccessible, not to mention shutting down their account. Can't blame this on ST. Took me a while to sort it out, but email kept coming and going for other users. I did stop/restart the SM service a couple of times, and had to do this during the workday today because of ongoing glitches in that user's account, but in a company with 40 users, this was acceptable. It didn't take long. And I don't use the A/V product that can be purchased with SM. I use an external cloud service for this filtering, in addition to basic O365 filtering.
**Setup for the split domain took some work, and it's not clear in SM just how to do this. Indeed, some of the settings are a bit backwards with respect to how I've got it configured, but it's all good. Working well for many months.