You receive a SMTP relay blocking error message when you try to send an e-mail


When you send e-mail messages you may receive an error message that your e-mail cannot be sent due to SMTP relay blocking. The exact error message may vary, depending on your Internet Service Provider (ISP); however, it will be 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 problem can occur with the following configurations:
  • You are logged in to a Local Area Network (LAN) that has an Internet gateway and attempt to send e-mail through an Internet Service Provider's SMTP gateway.
  • You are logged onto an Internet Service Provider (ISP) and attempt to send e-mail through another Internet Service Provider's SMTP gateway.
  • You are using a cable modem or ADSL to get to another Internet Service Provider and attempt to send e-mail through that ISP's SMTP gateway.
  • The reply address in the mail account properties is different than that of your Internet Service Provider.


This problem affects all messaging clients, regardless of manufacturer.

Customers should contact their ISP for assistance since this problem is not caused by the e-mail client. From a messaging client standpoint, there are no solutions for customers who have ISP's that block all SMTP Relay traffic.

Most of the new SMTP e-mail gateway software can allow relaying for specific IP addresses. This is how cable modem and ADSL users will be able to 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 will allow relaying for that specific IP address. Banks of IP addresses can also be allowed which may provide a workaround for some corporate customers who access ISP's through their corporate LAN.

If the error is random, it is possible that your SMTP server is checking whether the domain or the recipient name actually exists before sending. If the recipient's 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, no action is required. The message will be sent when the receiving server is again available.

More Information

This problem may become more common as ISP's begin to take action against SPAM e-mail. 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 uses an automated system to send commercial advertising, or mass marketing e-mail 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 in order to send e-mail 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 mail with two different Internet addresses but want the same reply address on both e-mail accounts. Other ISP's implement similar strategies.

In order to send or receive email through a LAN, additional ISP or DSL and similar connection devices through MSN, the smtp server settings must be set to secure. before the smtp server settings.

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


  • Employees have personal ISP accounts at where is your Service Provider's domain name. They use the company's corporate LAN to access their e-mail. They have no problems sending or receiving. One day they start receiving SMTP blocking errors when they try to reply to e-mail 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 e-mail through the LAN, but cannot send e-mail 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 is able to both send and receive mail from both accounts without problems. One day the user connects to, tries to send mail through and receives blocking errors. The user connects directly to and can still send and receive mail with both accounts. In this example implemented SMTP relay blocking and did not.

Article ID: 214402 - Last Review: Jun 22, 2014 - Revision: 1

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