I assume they're referring to TLS client certificate authentication support. As you say, most email clients can handle TLS. Few can handle authenticating to th

I assume they're referring to TLS client certificate authentication support. As you say, most email clients can handle TLS. Few can handle authenticating to the server with a client certificate, but FairEmail seems to support it fine, although I've yet to configure it on my own server as it's likely to break other clients that don't support it.