Recently after upgrading LS 3.7.4 to LS 4.0 (OS 10.12.5) I experienced problems sending email with two different email clients‚ Postbox and Airmail—outgoing email did not work at all. Initially I didn't suspect LS as being the source of the problems. Receipt of email wasn't affected. I used several utilities to sort out the problem. No solution.
Just before I was going to erase the SSD and do a clean reinstall of 10.12 I checked another Mac with 10.11 (and LS 3.7.4) installed on it. There were no such problems with sending email. So I checked and corrected the outgoing server settings on the 10.12 Mac to match those on the 10.11 Mac. Now sending email worked flawlessly. Then I again upgraded LS 3.7.4 to LS 4.0 and again the problem with outgoing server settings occurred. The changes imposed by LS 4.0 were to the Port (the ISP's required setting—Port 25—was changed to Port 587) and Security and Authentication (items not required by the ISP now showed as required).
So, is there any way that my settings (required by my ISP and/or by the email clients' developers) can be left unchanged by LS? Arbitrary alteration of such settings by LS is more than little annoying.
After an application of various utilities (Cocktail, DiskWarrior TechToolPro) I updated to macOS 10.12.6 this week. I then reinstalled LS 4.0 and found the previous problems did not recur. This Sierra update also apparently solved several other disparate problems unrelated to LS. So for the present anyway I have no more issues with Little Snitch