Unable to deliver mail to Yahoo, MSN and others alike Delivery to Yahoo.com and other large domain(s),are delayed and messages may be returned to the sender with an "I/O Error Encountered" after the retry limit is reached. There are a number of scenarios that may cause this problem:

1. The DNS settings are not configured correctly on the MailSite server. Yahoo will not accept messages if the reverse-DNS entry does not match the forward-DNS entry for the server. Yahoo will not accept messages if the HOST name in the SMTP transaction does not match the host name returned in the reverse-DNS record.

Example: mail.domain.com - IP = 10.10.10.1 (Forward DNS) / 10.10.10.1 - mail.domain.com (Reverse DNS)

The Related KB Doc explains how to correct this: 10076:Helo command rejected: need fully-qualified hostname




2. Many large mail services like Yahoo and others have started to use SMTP "tar pitting" to combat spam. As the volume of messages increases from a single host, Yahoo start to reduce the volume of messages it accepts in a single SMTP session.

   If this behavior continues, Yahoo will blacklist the host. A few of example situations that would cause this behavior are:

   • If you are hosting mail lists that contain many yahoo addresses.
   • If your mail users are sending messages out to many yahoo recipients.
   • A mailbox is auto-forwarding a MailSite mailbox to a Yahoo mailbox. Also if the user does not have any type of anti-spam or filters enabled, mass amounts of junk-mail/spam will be forwarded to yahoo.
     
     
3. You can determine whether these are causing the problem by reviewing the RCP files in the MailSite/Spool/Domains/ (Yahoo) folder(s)
4. You can restrict the maximum number of recipients in the MailSite Console: Server: Security: (Maximum number of recipients per message) to stop the first two of these problems.
5. Yahoo returns temporary error messages in certain situations where, according to current e-mail standards, they should not. These situations include, but are not limited to, receiving particular message types and when users have a full mailbox. We are not expecting these issues to be resolved in the near future by the large free email service providers, we however have come up with a workaround to these issues in our latest releases of MailSite Versions: 5.3.11 & 6.0.11
6. In the latest versions there have been significant improvements to the way MailSite deals with temporary errors, helping to reduce the affects of these problems. We strongly recommend to all our customers who are experiencing problems with Yahoo (or other large mail services e.g. Hotmail, AOL, MSN) to upgrade to the latest version of MailSite. Click to download MailSite 5.3.11 or 6.0.11

Summary of what was done in the latest versions of MailSite to workaround this issue:

If there are multiple messages for one domain (IE: Yahoo) MailSite will make a single connection to the remote host and being to process the messages in the order they where received. If one of the recipients triggers Yahoo to drop the connection, MailSite will then re-establish the connection and try again. The only problem with this is that the same message will be re-attempted for delivery which the connection will be dropped again. So to prevent this from happening which causes all the other "valid" recipients from being delivered properly, the "BAD" recipient will be moved to the bottom of the list and the rest of the recipients will be processed.