I went through this process just the other day, setting up Postfix on a Zerotier network. Postfix lets you specify a "mynetworks" config option that lets you al

I went through this process just the other day, setting up Postfix on a Zerotier network. Postfix lets you specify a "mynetworks" config option that lets you allow certain IP ranges. I added the Zerotier IPv6 assigned ranges to this list, disabled any form of TLS on the smtpd end, and was able to effectively turn off TLS. It was more annoying than I thought it would be but it makes sense given how few real world deployments lack TLS these days.