Only to one domain: Must issue a STARTTLS command first

[Hatrix]

Hello,

we have approx. 20K mails per day and most of the time all is ok, but we have one domain which makes problems:

XXXX@aexp.com

Their mailserver responds:
Action: failed
Status: 5.0.0
Remote-MTA: dns; wppim001.aexp.com
Diagnostic-Code: smtp; 530 Must issue a STARTTLS command first

So, I have no idea why it would fail for this mail connection. Any ideas on what to check? The problem is, as this is american express, we can not just send test-mails either, so it's a bit dificult for me to know exactly what's going on, any ideas?

best regards
Ray

[LMStone]

You should be able to send test emails to postmaster@<any domain> if the domain is configured correctly.

Are you trying to force TLS-only connections? Have you made any changes to Zimbra's default Postfix settings? What do your server's logs say during this transaction?

All the best,
Mark

[Hatrix]

Hi,

thanks for reply,

* TLS-only: NO
* Postfix config: no mentionable modifications
* Logs, nothing special, standard bounce message

I checked further and mails to aexp.fr go through, so I mailed postmaster so he checks on their side, just wanted to be sure it's no easy thing I forgot on our side,

best
Ray

[LMStone]

The following may help:

Postfix TLS Support

With best regards,
Mark