Slack has the ability to notify the current channel about sent messages. To do so, the user must mention "@channel" in his message. Pressing "@" activates auto-completion as usual but "channel" is not present there.  In addition, writing "@channel" down does not notify the channel. One must write "<!channel>" instead - I saw it by receiving messages for the entire channel sent by the official Slack client. Please make "@channel" work and add it in the completion list after @.