CAS uses Mailgun to send email messages to you & your customers. There are several reasons why outbound messages may fail to send. These are the most common causes.
First, identify if you can connect to Mailgun:
nc smtp.mailgun.org 587
You should see the netcat utility connect “220 Mailgun Influx ready”:
If connected, press CTRL+C to exit netcat.
If it doesn’t connect, then your outgoing port 587 is blocked.
Your firewall may be blocking outbound port 587 traffic, see: Configuring Server Firewalls
Digital Ocean has blocked your outbound mail traffic. Contact Digital Ocean.
Your server IP may be on a blacklist. Contact Mailgun.
If you have tried these steps and cannot resolve the problem, contact support.