Error: 550: 5.7.1 Unable to Relay when sending out POP3 messages

Based on what I see in various forums, there is a lot of confusion on setting up POP3 on Exchange Server 2007 in general. One common error that is mentioned a lot is:

550: 5.7.1 Unable to Relay

This is not the only scenario that you will see the above error. There are a lot of other scenarios as well. However, I will address one particular situation here.

Problem:

Users are unable to use Outlook 2007 or Windows Mail to send out messages (or reply to the messages they have received). They get the above error.

Explanation:

Here is the scenario:

You have a single Exchange Server 2007 running on a Windows Server 2003 Domain Controller. All the latest service packs and updates have been installed.

1. EXTERNAL USERS: You are able to receive e-mails sent to your POP3 accounts from the external users but you cannot send out e-mails to them, or reply to their messages. You get the “Unable to relay” error.

2. INTERNAL USERS: You are able to send & receive e-mails to other users on your internal network using POP3 from Outlook 2007, Windows Mail and OWA.

You are able to send & receive messages to internal & external users when using OWA because you are not using POP3 when you use OWA. This may be the temporary workaround that you are using because you can ‘t use POP3 to send out messages to external users.

Solution:

1. Go to Hub Transport under Server Configuration in Exchange Management Console.

2. On the Receive Connectors tab double-click the Default connector.

3. On the Permission Groups tab make sure that your Exchange Server has the following boxes checked:

– Anonymous users
– Exchange users
– Exchange servers

4. On the Authentication tab, make sure that you uncheck the box under Basic Authentication that says “Offer Basic authentication only after starting TLS.”

5. Restart Hub Transport service.

6. On the Outlook 2007 client go to the properties of the POP3 account. Click on More settings. On the Outgoing Server tab check the box “My outgoing server (SMTP) requires authentication”.

If you are using Windows Mail then the option is available on the Servers tab. Go to the properties of the POP3 account and check the box that says “My server requires authentication.”

Again, keep in mind this is not the only solution to this error. There are too many other possibilities and it’s almost impossible to cover them all but I am addressing one particular situation that you may find helpful.

Copyright ©2008 Zubair Alexander. All rights reserved.