Messages to Yahoo! Emails are Temporarily Deferred

This article applies to recent versions of SmarterMail. View articles for SmarterMail 15.x and earlier.
The growing concerns surrounding spam and harmful computer viruses have caused many mail servers to blacklist all inbound email sent from suspicious domains and servers. As a result, users can no longer accept messages routed through a potentially dangerous server.
While this feature may reduce or even eliminate spam from a particular domain, it can also create a communication barrier within your business. For example, it could result in delayed email messages or undelivered email messages. In fact, many SmarterMail users have received the following delivery error message when sending email to Yahoo!:

If you are seeing the error code “421 Message received from x.x.x.x temporarily deferred” or “451 Message temporarily deferred” in your SMTP logs, this indicates that the message you attempted to send was not accepted at Yahoo!. It is not a permanent error; your system will automatically re-try later.

If you are seeing this same error consistently over an extended period of time, and you feel that your policies and practices comply with our guidelines (described below), we would encourage you to contact us with detailed information so that we can help diagnose your problem.

For bulk mailers please visit ( ) to review our best practice recommendations and, if appropriate, request assistance.

If your mail server does not primarily send bulk mailings (e.g, you run a personal, corporate, educational, or ISP mail server), please visit ( ).

If you are not the administrator of the mail server in question, please contact the administrator directly with the error message you are receiving.

-- From Yahoo! Mail Help (

Yahoo!’s introduction of DomainKeysIM (DKIM), which combines DomainKeys and Identified Internet Mail (IIM), requires additional security measures to ensure safe email messaging. To prevent being "deferred" by Yahoo!, the following configuration changes for SmarterMail are recommended:

  1. Verify that a reverse DNS record is in place for all IPs that SmarterMail uses for outbound SMTP connections.
  2. Verify that an SPF record is in place for all hosted domains.
  3. Verify that DomainKeys/DKIM signing is enabled for your domains. (Versions 15.x and later no longer support DomainKey signing as that method for mail signing has become obsolete.)
  4. Use an outbound gateway with SpamAssassin and outbound filtering.

If this configuration does not resolve your deferment issue, you must become whitelisted by Yahoo! to ensure continued email delivery. To begin, you will need to fill out the appropriate form based on your email needs. If you send bulk email out on a regular basis, you will need to complete the Yahoo! Mail Bulk Sender Form. Alternatively, if you do not send out bulk mailings, you will need to complete the Yahoo! Mail Delivery Issues Form. Yahoo! may report that there are too many domains assigned to the same IP (Shared IP); however, with repeated requests, you will still become whitelisted.

Learn more about the SmarterMail secure business email server.


I am testing SmarterMail 14.5 and I have a couple of question if someone can assist me as I am new on this.

1. I have setup DKIM signing (according to instructions) and tested it with by sending out an email from the admin user and it all looks fine in the received email header. But when I do a DNS Test from the SmarterMail DKIM setup screen I get a Fail message. How can this be corrected ?

2. If I setup the a user in Outlook with the SMTP pointing to the SmarterMail server and send an email, the email is sent ok, but looking at the received email header there is NO DKIM signing. So what I see is happening that the Server is not signing the email if it comes thought it from a user not in the SmarterMail users list. Is this normal? How can we get this to work so the server is signing emails regardless from which user they are sent?

Constantine Serafim (3/8/2016 at 10:35 AM)
Hi Constantine! It sounds like there may be something wrong with the way DKIM is performing, as this isn't expected behavior. My recommendation would be to get in touch with our support team so they can look into the issue and find a resolution for you. Understand that if the issue is caused by a bug in the software, your support costs will be refunded. Here is where you can find more information on submitting an Email or Phone ticket: Let me know if I can help in any other way.
Andrea Free (3/8/2016 at 12:42 PM)
I have this issue, my email setup looks ok, but its been about a month now that the server hasn't been able to send to yahoo or AOL or similar addresses using the yahoo filtering system.
Any ideas on how to resolve the issues as Yahoo are unresponsive!

YS Tech (12/19/2019 at 3:50 AM)
Anthony -- verifying SPF and DKIM, and testing DMARC (and other DNS tests) is about the best suggestion we can make. MXToolbox has a good DNS Health Check tool ( and can point to problems that may affect email delivery. Beyond that, if the issue persists you may need to contact our support team.
Derek Curtis (12/19/2019 at 8:49 AM)