When suspended, the ESA appears unreachable for any communication trying to reach the suspended listener(s).
You can read more detail in my reply to another thread here : https://community.cisco.com/t5/email-security/when-outbound-private-listener-is-suspended-will-it-cause-queues/m-p/3690644
To confirm if the listeners are suspended, you can use listenerconfig from the CLI and it will show you which are suspended, or you can use the status command and review whether the System Status is Offline vs. Online.
mail.esalab.local> listenerconfig
Currently configured listeners:
1. INCOMING (on Data-1, 1.1.1.1) SMTP TCP Port 25 Public (SUSPENDED)
Thanks!
-Dennis M.