Please note that the following has nothing to do with Zen Cart Email Configuration or settings. There are no 'magic' changes that can be made to the shop Admin configuration to alleviate it, as it is completely external to the website setup.

The following will potentially affect anyone who uses a Yahoo Mail address in their website config or who has customers who use Yahoo Mail email addresses in the registration.

It appears that Yahoo are currently making changes in their mail server setup to more 'robustly' implement Domain Key checking of email.

See here:

http://www.newsfactor.com/story.xhtml?story_id=023002CUFRD8

http://mashable.com/2007/10/03/yahoo-mail-upgrades-spam-filter/


If you use, or have customers who use Yahoo Mail in the course of their purchases from your site, you should ensure that your website domain has a Domain Key record defined, and that your hosting company mail servers i.e. the mail servers you use to send your site email, are configured to apply Domain Key headers to the outgoing mail.
Mail without such headers is likely to be delayed, refused, or deleted.

The limited testing I've done suggests that forwarded mail is worst affected, which is potentially all mail from a website, but particularly 'Admin' copies of mail that are auto forwarded to a Yahoo mail address from a website domain address.
All such emails were either refused, or disappeared.

The simple solution is to refuse any registrations using affected email addresses, but more realistically, to implement a DNS domainkey record for your domain and to use a mail server that attaches domain key headers.