If you are using an email security layer called Mimecast or Proofpoint, you will not be able to connect that email to Follow Up Boss. Instead, choose a different email to connect for inbox syncing.
What are Mimecast and Proofpoint?
Mimecast and Proofpoint are enhanced security services for securing email accounts and communications for businesses.
How can I tell if I am using an enhanced security service?
If you are running into this message while connecting your email we have detected that you are using an enhanced security service:
We cannot connect to [email address] due to security settings. Use a different email for inbox syncing.
To proactively check if you are using an enhanced security service go to https://mxtoolbox.com/, enter your domain and choose MX lookup.