@mjg59 if you are talking about "advertising" your SMTP proxy service using MX records for your domain for an arbitrary sender, then that sender will by default use a clear text connection and optionally attempt to STARTTLS, but only if it is configured to use opportunistic TLS. Despite it being 2025, there is no guarantee that 100% of senders are configured to do this.
If you control the configuration of the originator of the email (the submitting client) and all of the hops that it will use to route from there to your proxy, then you may be able to configure SMTPS and maybe also SNI (it likely depends on how the TLS code was hooked up in the sender), but I have to stress that this is not common.
So unless you very specifically control the client and the server configurations, I'm not sure that you can guarantee the outcome you're looking for around this.