MailEnable will not send mail to remote mail servers


Explains the various reasons why MailEnable would be unable to send to remote servers, and the possible solutions.


When sending mail to a remote mail server the message is placed in MailEnable's SMTP outbound message queue. The status of this queue can be determined using the Administration Program (MailEnable Management->Servers->localhost->Connectors->SMTP->Queues->Outbound). Reasons why email is not being sent can be determined by viewing the SMTP Debug and Activity logs and within any bounce message that is returned to the sender address.


Below are some reasons that can cause problems when sending to remote servers.

1. The SMTP DNS Server has not been correctly defined or is experiencing problems resolving.

2. A firewall is blocking port 25 outbound or your ISP has banned outbound SMTP traffic.

3. Your IP address is listed in a spam database (Blacklist).

4. No PTR record (Reverse DNS record) defined for the outbound IP Address.


1. Ensure that the DNS server is working correctly and is able to resolve external domains using the Windows "nslookup" utility. Run the MailEnable Diagnostic Report. See: Article: and check for DNS resolution failures. The following articles can also help troubleshoot:

Diagnostic Report reports the error: 'Could not resolve your Local Domain Name in DNS':

Could not resolve MX list for domain appears in my debug log:

2. It is possible to verify if your ISP is blocking port 25 by attempting to telnet to port 25 of a remote mail host.

Windows telnet: telnet 25

If no response is received, then a firewall (either yours or your ISPs) is blocking outbound traffic on port 25. This can be validated by using the telnet utility to connect to your ISPs mail server (since they should at least allow connectivity to their server).

Eg: telnet {your ISPs Mail Server} 25

If you are able to telnet to your ISPs mail server, then this virtuallyconfirms that it is blocking outbound SMTP traffic. Ideally, MailEnable should be installed in an environment without this constraint. There is however a workaround, as follows:

Within the Administration Program, under the properties of the SMTP Connector, select the Smart Host tab and configure the properties of your ISPs SMTP Server. This will forward all outbound mail through their SMTP Server (which will in turn deliver it to the destination mail server).

Versions of macOS from Sierra onwards no longer provide the telnet utility. Therefore the alternative is to use SSH or Netcat.

1. Open "Terminal" utility on the mac.
2. Type:

nc 25 << EOF

HELO there



You will see an SMTP server response if the connection was successful indicating you are able to establish a connection with the remote SMTP server.

Example SMTP response:

220 ESMTP MailEnable Service, Version 10.39--10.39 ready at 03/28/22 10:30:00

More detail can be found within the following referencing article:

3. Please review the following article:

4. There are various DNS lookup tools that can be used to check a domains PTR record and if one exists. MailEnable provides a lookup tool that can be located here: another free site is:

Please see the following articles on how to obtain a PTR record:


Why are messages to external domains not being sent?

SMTP Connector Inbound Response Codes:

How to manually test if MailEnable can send mail to remote servers:

Using nslookup to check domain resolution:

What is a DNS PTR record?:

How to diagnose outgoing mail problems:

Product:MailEnable (All Versions)
Class:PRB: Product Problem or Issue
Created:7/03/2002 1:58:00 PM
Revised:Sunday, March 27, 2022