If you are using an email security layer called Mimecast or Proofpoint, you will not be able to connect your email to Follow Up Boss. Instead, connect a supported email provider 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?
Error when connecting your email in Follow Up Boss
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.
Domain Search
To proactively check if you are using an enhanced security service go to mxtoolbox.com > enter your email domain > MX Lookup.

If you see Mimecast or Proofpoint listed on this page, you are using an enhanced security service.

What do I do if I am using an enhanced security service?
If you are using an enhanced email security service such as Mimecast or Proofpoint, you will be unable to connect that email to Follow Up Boss. We recommend choosing a different email to use for inbox syncing. See our list of recommended email providers here.