POP Before SMTP troubleshooting including - 503 Error on relay


Using POP authentication to authorize relay through a server can be inhibited by the environmental issues.


How does POP before SMTP work in MailEnable?

When a POP authentication is approved by the server by completing a 'receive' from a client, then a simple file is created within the Program Files\Mail Enable\Config\Connections directory. This file is a *.PBS extension and is named as the connecting IP addrress.

For troubleshooting purposes, ensure this file is being created for the testing account.
The information MailEnable uses from the file is name (or IP address) and the modified date. The modified date is compared against the time allowed for retaining the authentication. If the relay happen inside of this allocated time, then relay is allowed. If the relay happens outside of this allocated time, then the relay is denied.
Secondly, check the time stamp on the file against the POP before SMTP relay time setting and ensure the file is being updated each time it is accessed.

Dial-up and large ISP port blocking
Using dial up and some ISPs can prevent the usage of POP before SMTP authentication, as the port 25 has been blocked for all SMTP traffic except through the ISPs service.

Dial up IP change can affect POP before SMTP relay.
Another problem that can occur with dial up is due to connecting IPs changing for each connection.  With a dial up connection, a connection is made through POP and mail retrieved, or authentication credentials passed and the PBS file is created and named after the connecting IP address.  The problem occurs when the next connection is made for SMTP sending or relay. With dial up, it is very likely that a new connection will have a new IP on the client side which will be rejected for relay, as the IP verification is not matched due to a different IP address being used.

Proxy or firewall
If the problem is occuring with clients across multiple domains, then there is a chance that a proxy or firewall is blocking the port or redirecting the traffic.  This can especially be an issue with Cisco routers as they intercept all SMTP requests using a feature called MailGuard. This service will disallow authentication even if the client and server sides are configured correctly for relay or authentication.  For information on a solution to this issue please check the following link:

Antivirus firewalls

Some antivirus firewalls can stop the authentication. To verify this is the issue, disable the antivirus firewall before authenticating to see if the problem is still occuring.  If the antivirus firewall is causing the issue, then either remove/disable the firewall compenent of the software or configure the firewall to allow the desired traffic.

Extra client diagnostic logging feature
A useful tool for diagnosing client - server communication from the client side is the Outlook Express Maintenance feature.

To enable Outlook Express Maintenance;

1. Open Outlook Express
2. Open Tools
3. Click on Maintenance Tab
4. At the bottom of this tab is a troubleshooting section. When a mail protocol is enabled in this view, all client access using this client will be logged into a [protocol eg.IMAP.log] file.
5. The location of the log files can be found by clicking on the Store Folder button above the troubleshooting section.

Here is an example - C:\Documents and Settings\User\LocalSettings\Application

Here is an example of a log output file of a client communication session;

Outlook Express 6.00.2800.1437
SMTP Log started at 08/25/2004 15:15:29
SMTP: 15:15:30 [rx] 220 example.com ESMTP MailEnable Service, Version:
0-1.2- ready at 08/25/04 07:11:30
SMTP: 15:15:30 [tx] EHLO mesrv3
SMTP: 15:15:30 [rx] 250-example.com [218.xxx.xx.xxx], this server offers
2 extensions
SMTP: 15:15:30 [rx] 250-SIZE 10000000
SMTP: 15:15:30 [rx] 250 HELP
SMTP: 15:15:30 [tx] MAIL FROM: <
SMTP: 15:15:30 [rx] 250 Requested mail action okay, completed
SMTP: 15:15:30 [tx] RCPT TO: <
SMTP: 15:15:31 [rx] 250 Requested mail action okay, completed
SMTP: 15:15:31 [tx] DATA
SMTP: 15:15:31 [rx] 354 Start mail input; end with .
SMTP: 15:15:31 [tx] .
SMTP: 15:15:32 [rx] 250 Requested mail action okay, completed
SMTP: 15:15:32 [tx] QUIT
SMTP: 15:15:32 [rx] 221 Service closing transmission channel

Outlook first time send fail

Outlook attempts to send email before using POP to check for email. Therefore if a message is being sent and the user has not authenticated via POP it will fail to send the message. Having the client attempt to send the message the second time will succeed because the POP authentication would have been performed.


Error '503 This mail server requires authentication': http://www.mailenable.com/kb/content/article.asp?ID=ME020135

Product:MailEnable (All Versions)
Class:TRB: Troubleshooting (Configuration or Environment)
Revised:Wednesday, May 4, 2016