I second the initial query. Here is my use case: we have set channels for workgroups, that usually have 3-10 users maximum. Getting an email notification is really important for us when someone posts something new, because some of the workgroups are on a long-term slow moving process.
Right now it is kind of ridiculous, as people have to remember to tell everyone they have posted something, which defeats the purpose in the first place!
The behavior I really wish mattermost had is precisely that of an automated @channel notification that users can enable either globally (i.e. get an email each time someone posts in a channel I am registered in), or on a per-channel setting.
This is no spamming at all since users (and I) are asking for the option.
Admittedly, this might not be a need if/when people are regularly logged in, which is NOT the case for everybody.