E2E Email Testing

Something I&rsquo;ve been wanting to do for a while: end-to-end testing of my email&#xA;infrastructure. A recent incident offered the opportunity. &#xA; The incident &#xA; Some days after re-installing servers to the latest FreeBSD version, some&#xA;acquaintance on Slack was kind enough to report that their messages to me were&#xA;not delivered: