How to configure tls_options for safe direct mail delivery (without dedicated relay)

[mmzeeman]

With OTP 26 the ssl application's new default is {verify, verify_peer}. This option only works when one passes certs (or use verify_none).

For secure delivery one has to check the certificate chain and the name of the smtp server by passing a verify_fun in the tls_options.

One problem with this is that a user of the library does not know the name of the smtp server without doing a DNS MX record lookup. This is however done inside gen_smtp, so it can't be configured by the user of the library (without also doing a MX lookup outside gen_smtp)

Is it currently possible to validate the name of the smtp server via a verify_fun so one can safely deliver email to the correct mailserver? Or are changes needed?