The top reason why a user's messages are not being sent is an "unsynced" mailbox or social media profile. Here are some of the reasons for that:
1. You never synced your mailbox or social media account
Sync your mailbox and social media account with hubsell: https://help.hubsell.com/en/articles/4185080-which-basic-settings-do-i-need-to-make-to-get-started
2. You recently changed your email password
When you change your email or social media account password it voids the token that is provided by your ESP (email service provider) or the social network.
How to solve: follow exactly the same steps as point 1 "You never synced your mailbox"