by disabling pipelining on the external port.
The fix in the postfix config for the smtp-smuggling vulnerability [0]
follows the current recommendation of postfix upstream [1].
by using `smtpd_data_restrictions` instead of the newer
`smtpd_forbid_unauth_pipelining` the fix works for both PMG 7 and 8.
Tested with a handcrafted smtp-smuggling-session and verifying that:
* without the fix I get 2 mails
* with the fix I get 1 mail when sending to the external port, but
still 2 mails when sending to the internal port