Using POP authentication to authorize relay through a server can be inhibited by the environmental issues.
How does POP before SMTP work in
Dial-up and large ISP port
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: Article ME020159
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.
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.
is an example - C:\Documents and
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
SMTP: 15:15:30 [rx] 250-SIZE 10000000
SMTP: 15:15:30 [rx] 250 HELP
SMTP: 15:15:30 [tx] MAIL FROM: <email@example.com>
SMTP: 15:15:30 [rx] 250 Requested mail action okay, completed
SMTP: 15:15:30 [tx] RCPT TO: <firstname.lastname@example.org>
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': Article ME020135
|Product:||MailEnable (All Versions)|
|Keywords:||PBS SMTP POP 503 relay before trouble troubleshooting authentication error|
|Class:||TRB: Troubleshooting (Configuration or Environment)|
|Revised:||Wednesday, May 4, 2016|