The Sucuri Firewall functions as a reverse proxy. A simple change to your DNS settings is all it takes to thoroughly filter incoming traffic to sniff and parse out bad requests from the good ones. However, these DNS changes can affect connectivity to your mail service if not properly implemented.
In fact, this is a very common issue that is regularly raised with our support team. The Sucuri WAF is not designed to support or filter your email — and many users either intentionally or accidentally try to send email to the firewall for filtering, resulting in various problems.