Mail notification subject contain channel's name
The email is sent when someone who are in channel mentioned.
But, there is no information about channel name in email notification.
(cf. push notification has channel name information on it.)
For better identification, I think that email notification need to provide channel name to mail subject.
This feature is already merged into master and will be available in v3.4. Pull request here: https://github.com/mattermost/platform/pull/3824