SMTP Relay
Mail server variant for any SMTP-capable MTA such as Postfix, Sendmail, Zimbra or Kerio. Smart host plus header-based loop protection.
Prerequisite: Step 1, Domain & DNS Setup is complete and all four DNS checks are green.
What happens here: The MTA (Postfix, Sendmail, Zimbra, Kerio or similar) is configured to use Conbool as its smart host. Inbound connections from Conbool are allowed. Loop protection and spam classification are added in the MTA's own rule language. In the Setup Assistant, the Manual configuration mode is chosen.
Effort: 30 to 60 minutes.
Step A. Outbound Smart Host
In the MTA, set the following routing:
| Field | Value |
|---|---|
| Relay host or smart host | mail.conbool.com |
| Port | 25 |
| Authentication | None, TLS-certificate based |
| TLS | Required, domain validation against mail.conbool.com |
| MX lookup | Disabled |
| Scope | All hosted domains, or specific domains |
The typical knobs per MTA:
- Postfix:
relayhost = [mail.conbool.com]:25,smtp_tls_security_level = encrypt,smtp_tls_secure_cert_match = nexthop - Sendmail:
define(\SMART_HOST', `mail.conbool.com')with activeSTARTTLS` and certificate validation - Zimbra: Global Outbound SMTP settings → smart host
mail.conbool.com, TLS = Required - Kerio Connect: SMTP server → Relay SMTP server →
mail.conbool.com, Use SSL/TLS
Step B. Allow Inbound Connections from Conbool
- Allow port
25for SMTP only from the Conbool IP addresses, i.e. firewall plus MTA restrictions. - Set the recipient whitelist to the hosted domains.
- Require TLS for inbound connections, accept the certificate name
mail.conbool.com.
Step C. Loop Protection and Spam Classification
In the MTA's rule language (header check, routing rule, filter plugin), implement:
- Messages with
X-Conbool-<YourDomain>: trueare not sent through the smart host but delivered directly. - Messages with
X-Conbool-Flag: YESgo to the Spam folder, e.g. set an SCL equivalent or addX-Spam-Flag: YES. - Calendaring messages and messages with
Return-Path: <>are excluded from smart-host routing.
Done When
Back in the Setup Assistant at the Mail Server step. Enter the relay host of the inbound MTA, for example mx-internal.yourcompany.com, then click Test Connection. Status "SMTP reachable" means the connection is established.
Continue with the connection test and completion.