Accepting only encrypted mails from a defined domain

Hi,

i'm trying to configure TLS between two sites: my (A) and customer (B).
I can force the sending of email from A->B using the directive:
smtp_tls_per_site = hash:/etc/postfix/tls_per_site
B.domain MUST

in addition to the specular config on B (so that B->A is forced to TLS),
i want configure A to accept mails from B only if they are encrypted.
i can't' find a solution to this problem.

i'm wondering if there is a directive "specular" to:
smtp_tls_per_site = hash:/etc/postfix/tls_per_site

constrains:
- only traffic between A<->B must be forced to encryption. communication
with other servers must be possible in clear text.

thanx for help
rcc

> in addition to the specular config on B (so that B->A is forced to TLS),
> i want configure A to accept mails from B only if they are encrypted.
> i can't' find a solution to this problem.
smtpd_tls_auth_only = yes