Plesk 12.x / Linux :: Recurring Listing On CBL For Using Several Different EHLO / HELO Names

I've been plagued by CBL listing for quite some time now, on a linux server with Plesk 12.After months of a fierce fight against every possible malware on the about 120 various websites on this server, extensively monitoring clients emails, enabling restrictive policies and finally even hiring a private security firm to investigate the problems further, we were sure that not a single spam message was sent by our server in any way.

So we finally contacted CBL, exposed the issue and got this answer:The CBL attempts to detect compromised machines in a number of ways based upon the email that the CBL's mail servers receive.During this it tries distinguish whether the connections represent real mail servers by ensuring that each connection is claiming a plausible machine name for itself (via SMTP HELO), and not listing any IP that corresponds to a real mail server (or several mail servers if the IP address is a NAT firewall with multiple mail servers behind it). 54.194.XX.XXX was found to be using several different EHLO/HELO names during multiple connections on or about:

2015:01:09 ~16:30 UTC+/- 15 minutes (approximately 3 days, 21 hours, 14 minutes ago).

The names seen included: xxx1.xx, xxx2.xx, xxx3.xx, xxx4.xx, xx.xxx5.xx, veniceberg.com..Note that the above list may include one or more names that are not fully qualified DNS names (FQDNs). Host names (ie: Windows node names) without a dot are not FQDNs.

The final possibility is that 54.194.XX.XXX is not a NAT firewall, and is instead a single box with many domains provisioned on it, some that send email directly, setting the HELO as the sending domain. If this is the case, to prevent a relisting we strongly recommend setting the mail software on the box so that a single identifying name is used in outbound SMTP connections mail software on the box so that a single identifying name is used in outbound SMTP connections. As an alternate workaround, you can configure the mail software to relay its outbound email through an intermediate mail server. Even a co-resident mail server package (such as IIS on Windows) will do fine.​

This pointed me to this Plesk Mail setting (not sure if this selection is the default). Now we are waiting a few days to see if changing to "Send from domain IP addresses" solves the issue. I think this is a kind of issue which deserves attention by Parallels to avoid other users go trough our fatiguing ordeals. If this setting is responsible for getting servers blacklisted, it should be highly discouraged.