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.