[SOLVED] Lost connection while receiving the initial server greeting

[molybtek]

While trying to set up a smart relay, I've followed the instructions found at
Outgoing SMTP Authentication - Zimbra :: Wiki

I've ran the following (where mx.mail.com is replaced by the mail host I'm using):

I've created the relay_password file, and ran postmap on it.

zmprov ms server.domain.com zimbraMtaRelayHost mx.mail.com:587

postconf -e smtp_sasl_password_maps=hash:/opt/zimbra/conf/relay_password
postconf -e smtp_sasl_auth_enable=yes
postconf -e smtp_cname_overrides_servername=no
postconf -e smtp_use_tls=yes
postconf -e smtp_sasl_mechanism_filter=plain,login
postconf -e smtp_sasl_security_options=noanonymous
postconf -e smtp_tls_security_level=may

postfix reload

However, even after doing all those, I'm still getting the error lost connection with mx.mail.com[xxx.xxx.xxx.xxx] while receiving the initial server greeting

I've tried sending via thunderbird with this host and i am able to send email: using port 587, secure authentication unchecked, Connection Security: STARTTLS

Are there any way I can get more details on what is failing? Thanks.

[molybtek]

Forgot to mention, the mail server I'm trying to connect to is running MS Exchange.
I'm able to use the same settings to connect successfully to gmail, but still no luck with Exchange. I've removed the relay passwords settings and still get the same error - so I'm guessing it failed even before the authentication...