In the upper right hand menu, you can see what's been happening in your account.
When you click on the notification symbol, you can see all of your notifications. They can be marked all as read, marked as read and deleted.
You will see notifications when:
Comments
Another user comments in the 'Collaborate' section.
Another user replied to your comment in the 'Collaborate' section.
Multiple people comment on your email in the 'Collaborate' section.
When you are tagged (@your.name) in a comment in the 'Collaborate' section.
Status of emails
Your email has been approved.
Your email is in review.
Your email has been deleted.
Requests
A user has requested your approval for an email.
A user has requested changed to your email.
Account invitations
Team members have accepted your invitation to their account.