==== Required information ====
- iRedMail version (check /etc/iredmail-release):  iRedAdmin-Pro-LDAP-2.6.0
- Linux/BSD distribution name and version: Ubuntu 16.04 LTS
- Store mail accounts in which backend (LDAP/MySQL/PGSQL): LDAP
- Web server (Apache or Nginx): Nginx
- Manage mail accounts with iRedAdmin-Pro? Yes
- Related log if you're reporting an issue:

Resolving hostname...
Connecting...
Connection: opening to exchange.say-digital.io:25, timeout=300, options=array (
                   )
Connection: opened
SERVER -> CLIENT: 220-exchange.say-digital.io ESMTP Postfix
                   220 exchange.say-digital.io ESMTP Postfix
CLIENT -> SERVER: EHLO tools.wormly.com
SERVER -> CLIENT: 250-exchange.say-digital.io
                   250-SIZE 15728640
                   250-ETRN
                   250-STARTTLS
                   250-ENHANCEDSTATUSCODES
                   250-8BITMIME
                   250-DSN
                   250 SMTPUTF8
CLIENT -> SERVER: STARTTLS
SERVER -> CLIENT: 220 2.0.0 Ready to start TLS
CLIENT -> SERVER: EHLO tools.wormly.com
SERVER -> CLIENT: 250-exchange.say-digital.io
                   250-SIZE 15728640
                   250-ETRN
                   250-ENHANCEDSTATUSCODES
                   250-8BITMIME
                   250-DSN
                   250 SMTPUTF8
CLIENT -> SERVER: MAIL FROM:
SERVER -> CLIENT: 250 2.1.0 Ok
CLIENT -> SERVER: RCPT TO:
SERVER -> CLIENT: 550 5.7.1 Service unavailable; client [96.126.113.160] blocked using zen.spamhaus.org
SMTP ERROR: RCPT TO command failed: 550 5.7.1 Service unavailable; client [96.126.113.160] blocked using zen.spamhaus.org
CLIENT -> SERVER: QUIT
SERVER -> CLIENT: 221 2.0.0 Bye
Connection: closed
2016-09-29 10:17:54    SMTP Error: The following recipients failed: test@saydigital.io: Service unavailable; client [96.126.113.160] blocked using zen.spamhaus.org
Message sending failed.
====

Hello i got some trouble with the fresh Iredmail install
i plan to use 2 mails server ( one for our company and one the iredmailPro for my customers )
i successfully setup both
Ours: mail.say-digital.io Works fine
Customers: exchange.say-digital.io
when i sent a message no problem works as well
when i sent a message to a mail box with the mx exchange.say-digital.io i got:
Message sending failed

I thinks its from my DNS configuration:
i have

;; MX Records
say-digital.io.    300    IN    MX    10    mail.say-digital.io.
exchange.say-digital.io.    300    IN    MX    10    exchange.say-digital.io.

;; A Records (IPv4 addresses)
mail.say-digital.io.    300    IN    A    165.172.XX.XX

exchange.say-digital.io.    300    IN    A    164.172.XXX.XX

say-digital.io.    300    IN    A    174.127.XXx.XX