You receive a SMTP relay blocks error message when you send e-mail messages in Outlook

For a Microsoft Outlook 2000 version of this article, see 214402 .

For a Microsoft Outlook 98 version of this article, see 217526 .

For a Microsoft Outlook 97 version of this article, see 217534 .


When you send messages, you may receive an error message that your message cannot be sent because of Simple Mail Transfer Protocol (SMTP) relay blocking. The exact error message may vary, depending on your Internet service provider (ISP). However, it is similar to the following error message:
The message could not be sent because one of the recipients was rejected by the server. The rejected e-mail address was '<>'. Subject: '<Test>', Account: '<Test>', Server: '<>', Protocol: SMTP, Server Response: '550 <>... Relaying Denied', Port: 25, Secure (SSL): No, Server Error: 550, Error Number: 0x800CCC79


This behavior may occur if you have one of the following configurations:
  • You are logged on to a Local Area Network (LAN) that has an Internet gateway and you try to send messages through an ISP's SMTP gateway.
  • You are logged on to an ISP and you try to send messages through another ISP's SMTP gateway.
  • You are using a cable modem or ADSL to get to another ISP and you try to send messages.

This behavior affects all messaging clients, regardless of the manufacturer.


To resolve this behavior, contact your ISP for help. The e-mail client does not cause this behavior. From a messaging client standpoint, there are no solutions for customers whose ISPs block all SMTP Relay traffic.

Most of the new SMTP e-mail gateway software permits relaying for specific IP addresses. This is how cable modem and ADSL users can continue to use their same ISP. The cable modem or ADSL provider must provide the customer a static IP address, in turn, the customer's e-mail ISP allows relaying for that specific IP address. Banks of IP addresses can also be permitted, which may provide a work around for some corporate customers who access ISPs through their corporate LAN.

If the error message that is described in the "Symptoms" section of this article is random, it is possible that your SMTP server is checking whether the domain or the recipient name actually exists before sending the message. If the recipient's e-mail server is temporarily down or unavailable for another reason, such as high network volume, your SMTP server cannot verify the address and may reject the relay. In this case, action is not required. The message is sent when the receiving server is available again.

More Information

This behavior may become more common as ISPs start to take action against SPAM messages. SPAM is flooding the Internet with many copies of the same message in an attempt to send the message to people who would not otherwise choose to receive it. Most SPAM messages use an automated system to send commercial advertising, or mass marketing messages. SPAM costs the sender very little to send; most of the costs are paid for by the recipient or the e-mail providers.

Some ISP's are fighting SPAM by requiring each user to be dialed directly into their system to send messages through their SMTP gateway. This is an easy way to control SPAM, but may affect some users who tunnel into their e-mail account from another ISP.

MSN, the Microsoft Network, is one example of an ISP that has imposed SPAM blocking. Additionally, MSN has imposed restrictions whereby if the e-mail address in your Internet Account properties does not match that of your MSN account, you may also receive SMTP blocking errors. This affects users who receive messages with two different Internet addresses, but that want the same reply address on both e-mail accounts. Other ISP's implement similar strategies.

To send or receive messages through a LAN, additional ISP or DSL and similar connection devices through MSN, the SMTP server settings must be set to secure.

MSN e-mail server settings for a LAN, DSL or third-party connection must be set to

Examples of SMTP Relay Blocking

  • Employees have personal ISP accounts at where is your Service Provider's domain name. They use the company's corporate LAN to access their messages, and they have any problems sending or receiving. One day they start receiving SMTP blocking errors when they try to reply to messages that they have received from They contact technical support and are told that just installed new SMTP gateway software which prevents SMTP relaying. Now the employees can receive messages through the LAN, but cannot send messages because they're not connected directly to
  • A user has two ISP accounts, one with and one with, where and are the Service Providers' domain names. The user dials into and can send and receive messages from both accounts without problems. One day the user connects to and tries to send messages through and receives blocking errors. The user connects directly to and can still send and receive messages with both accounts. In this example, implemented SMTP relay blocking and did not.