CAS uses the SMTP server provided by 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.
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Your firewall may be blocking outbound port 587 traffic, see: Configuring Server Firewalls
Digital Ocean has blocked your outbound mail traffic. Contact Digital Ocean. Make sure that you mention your use case, and that you are a GB Operator. Speak to a live human about it.
Your server IP may be on a blacklist. Contact Mailgun.
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