SMTP proxy with "STARTTLS only" accepts unencrypted mail

[Maxim Dounin]

Hello!

On Mon, Jun 03, 2019 at 10:16:20PM +0200, Marcus wrote:

> I try to use NGiNX 1.10.3-1+deb9u2 (Debian 9 version) as SMTP proxy in 
> front of a postfix server. I defined one server that should accept 
> encrypted connections only. Therefore I set "starttls only".
> 
> But this server accepts plaintext mails also. If I use telnet to test 
> the proxy it provides STARTTLS but I can relay a mail without using it.

Currently, "starttls" is only enforced for authentication, but not 
for non-authenticated mail delivery, as with "smtp_auth none" in 
your config.  If you want to reject all non-encrypted 
non-authenticated mail delivery, you can do so using an auth_http 
script.

Recently discussed here:

http://mailman.nginx.org/pipermail/nginx-devel/2019-March/011970.html

-- 
Maxim Dounin
http://mdounin.ru/