Are you using Let’s Encrypt with MDaemon or SecurityGateway (or anywhere else)? If so, great! But due to a bug re-validating CAA records, Let’s Encrypt will be revoking a subset of otherwise valid certificates. This bug has existed since 2019-07 and therefore could apply to any certificate issued prior to the fix which was applied 2020-02-29.
So what should you do? Well, luckily there is a tool to check your certificate, so you should check to see if your certificate is being revoked and if so, issue a new certificate as quickly as possible.
Modern browsers don’t check certificate revocations immediately or on all requests, so just because your browser works does not mean there is no impact! If your certificate is revoked you may see an impact some time in the next week or so, or you might not see it at all while users of other operation system / browser / client combinations may have a different experience.