I hope this post helps people find a problem that they - like me - didn't know they had.

I would recommend everyone to check the SMTP logs for "Client socket is disconnected! Disconnect exception encountered: False, IsDisconnected: True".

We have several customers who did not receive certain messages. One customer alone did not receive over 50 emails in a one month period - all senders were Office365 users. The problem did not occur regularly but irregularly. One message from the sender arrived successfully, the next again not - without pattern. This probably also led to the fact that no customer noticed it immediately. Not receiving emails on this scale is not acceptable for any customer. And who is to blame? The customer is not interested. From his point of view, the product we offer him is unreliable. And the fact that besides mass mailers also Office365 is partially locked out does not make our point of view any more credible nor does it make the customer's problem any smaller, who now has to write to a large number of people to ask them to please deliver the message to them again.

Since this was only possible because SmarterTools enabled an option as the new default for everyone, an option that isn't even listed in the changelog, here are my thoughts on it based on a post by Tim U. on the subject that I found while searching for the error message:

Regarding "If you want to call this a win, call it a win. You will be opening yourself up to duplicate messages and more."
Was I not open to the problem of duplicates until now? And even if I was, should I call it a "win" when customers no longer receive normal emails from Office365 servers? I can't do Microsoft's homework. I need to be able to provide a working, reliable product. That's a "win", that's what I make money on, that's what I pay SmarterTools money for.

Regarding "How SmarterMail works today will continue to be the default because it is correct."
The wrong answer to the wrong question. The question is not what is correct. The question should be "what works, what is stable, what is reliable?". I find it irresponsible to introduce such an option and at the same time activate it over the decision of us customers without first having carefully examined the consequences.

I wish such measures would be rolled out more gently. Don't switch it on as default and then offer an option in the UI to disable it some releases later because customers demand it. But add an option first, work with larger customers and see what happens when they activate the option, etc.

I received the following response from support (whom I do not want to criticize and is always friendly and helpful): 

But if this is known to SmarterTools, surely I expect proactive information about it? As a non-SmarterTrack customer I even get notifications about maintenance windows that don't affect me but that an option that SmarterTools simply enables by default might lock out servers of big providers like O365 I am not informed about but have to wait until customers complain to me to contact SmarterTools and then learn that I better disable the option then? This is just totally customer unfriendly and reactive instead of proactive. If I had been informed, I could at least have decided for myself or check the logs regularly for this case.

I am sorry to say this but like many before me here in the community, I sometimes feel more like a test subject than a customer. In this respect, we SmarterMail customers are clearly in the weaker position, because even if our end customers leave us due to dissatisfaction or lack of stability of SmarterMail: as long as we still have active customers, we have to maintain our SmarterMail licenses.

The battle against O365 is tough enough otherwise. Why not listen to the community and start implementing some of the things others suggested here? For example, beta's or a listing of known issues at least for paying customers? I've spent countless amounts of time on bug reports to support where the response has been that SmarterTool already knows about it and is working on a solution. Thank you, if only I had known this sooner and could have just joined "the list" and then gotten a custom build instead of spending all that time. We are not talking about about software for a snack machine, it's about email - for probably every company one of the most important tools in their daily work (which is why I also totally don't get your 4 day week idea to be honest).

We want to offer our customers excellent service with top products and with no product do we stand behind our own requirements as we do with SmarterMail. With no customer group have we had to apologize as often as with the customers who use SmarterMail. That just makes you think. The market has become so tough with O365 and the advertising Microsoft is doing in Windows to get even more customers that we can't compete with alternative offerings unless they are outstandingly good. I believe in the idea of SmarterMail but the quality and the decisions made just have to get to a level that allows us to continue to offer our customers exciting and good solutions that are reliable.

As a small example to conclude: I don't do updates anymore, I wait and watch what others report and then maybe do an update if I have to. The thought about the daylight saving time change already makes me nervous, as there have been several cases in the past where calendar entries of customers were no longer correct - and that already with more than one daylight saving time change. These are all thoughts that, if SmarterMail is as reliable as SmarterTool likes to present it, I shouldn't have so present.