Right now it’s not possible to use DNS over TLS in Stalwart when not selecting the provided options like Quad9. There is already an option to define custom DNS over TLS resolvers but when using a FQDN - which is required to verify - this does not work and requires a database/feature change.
It would be great to be able to set custom DNS over TLS resolvers with FQDN.
Previous discussion: DNS over TLS Configuration