Send email notification to colleague when he/she is mentioned while being offline.

Thanks for sharing this idea Tim! I'm keeping track of this wish and keep you up to date if there is any news. 

A slack notification to that colleague would also be a nice feature

Would be a great feature so make it optional for users to receive email notification when they get mentioned. Not only when they are offline.

We are interested in implementing this feature. Our suggestion is a "notification manager" for every user. This way you can chose if, how often and where you would like to receive these notifications. 

At this moment this feature is not yet planned. However we do feel it is very interesting, but are looking to investigate a possibility to enable / disable email notifications for all kinds of incoming / outgoing messages, reminders, etcetera.