Mail client can send locally but is unable to send to remote mail servers


A mail client can send to locally serviced domains on the MailEnable server but not to remote domains.

Error returned by mail client when unable to send to remote servers:

The message could not be sent because one of the recipients was rejected by the server. The rejected e-mail address was 'user@domainname'. Subject 'test', Account: 'localhost', Server: 'localhost', Protocol: SMTP, Server Response: '503 This mail server requires authentication when attempting to send to a non-local e-mail address. Please check your mail client settings or contact your administrator to verify that the domain or address is defined for this server.', Port: 25, Secure(SSL): No, Server Error: 503, Error Number: 0x800CCC79


The most likely reasons for this follow;

  1. MailEnable has not been configured to allow relay for authenticated users or the mail client has not been configured to authenticate against the MailEnable SMTP connector
  2. The AUTH Extension was not reported in the EHLO response from the server because the extension or verb for AUTH or EHLO have been disabled
  3. The mail client is configured to connect through an SMTP proxy or firewall and is not directly hitting the mail server at all. (E.g. Cisco PIX firewalls may use 'Mailguard' which does not support SMTP AUTH.  Therefore either disable the Mailguard feature on the firewall or use POP before SMTP).
  4. The mail client does not support SMTP Authentication.


Here are some possible solutions:

  1. Ensure AUTH and EHLO are enabled; SMTP extensions in the SMTP properties under the "Advanced SMTP" tab.
  2. Review the following article on how to configure the CISCO pix firewall to allow correct authentication:
  3. Mail client program does not support authentication. This is very unlikely, unless using an old version of a mail client program. A list of compatible mail clients is available in the following article:
  4. Ensure the mail client has been configured correctly and "My server requires authentication" has been enabled in the account properties if using Microsoft Outlook or Outlook Express. More information on how to correctly setup your mail client software can be found in the following article:

Here is some detailed information relating to the process that occurs when a mail client connects to MailEnable and attempts to authenticate:

  1. When the client connects to the server, the server sends the client a welcome message.
  2. Then the client sends a EHLO command to the server and the server will respond with a list of its supported features.
  3. This is the default behaviour and if it is not the case, then somehow the SMTP extension for EHLO or AUTH has been disabled. Ensure that a firewall is not intercepting the EHLO or AUTH request from the client, hence preventing SMTP authentication.


POP Before SMTP troubleshooting including - 503 Error on relay:

Diagnosing POP and SMTP Authentication Problems:

Error '503 This mail server requires authentication':

Product:MailEnable (All Versions)
Class:PRB: Product Problem or Issue
Revised:Wednesday, May 4, 2016