Issue: @mention emails are not sending notifications to the respective users.

Hi all,

I used my profile(Admin) and my colleagues profile (External user) to test if she was receiving emails when ever I mention her on a community group or profile and vice versa. Either of us received emails. From documents I made sure of the following, 

• Enable Email notifications on community
• Checked all of the other email settings including "Mentions me in a post" and "Mentions me in a comment"
• Internal org has the chatter settings enabled

But none of this seems to be working. There is lot of posting happening in our community, unfortunately people are not getting notified and hence answering or replying to the posts is getting delayed. Any help or suggestion on this would be highly appreciated. 

Thanks,

Abhinandana G