Update 5/1/2014: AOL has also updated their security settings, which will cause messages to be rejected. See the following post by AOL: http://postmaster-blog.aol.com/2014/04/22/aol-mail-updates-dmarc-policy-to-reject/
Recently, Yahoo made a change to their domain email policy that is causing a large amount of email messages from being rejected.
Here are a few articles we came across while researching this issue:
This issue is causing issues for RescueGroups.org clients in the following ways:
Organization Email Address
If your organization's email address is a Yahoo address (@yahoo.com) or AOL address (@aol.com), many email providers will reject the emails that are sent by our system.
Problem: When our servers send an email to one of your visitors (primarily with our Website service), our server uses your organization email address as the source for the message. If the source email address is on the Yahoo domain the message is rejected due to Yahoo/AOL's security setting.
Resolution: Change your organization's email address to one that isn't on the Yahoo or AOL domain. You can do that by going to Services > Account Administration > Contact Information.
Inbound email from @yahoo.com and @aol.com email addresses will be rejected when we try to forward them on to you.
Problem: When our email service forwards an email to your final destination mailbox, our server uses the source email address as the source for the forwarded message. If the source email address is on the Yahoo or AOL domain the message is rejected due to Yahoo's security setting.
Resolution: We do not have a resolution to this issue other than switching to email hosting (and not using email forwarding).
Online Forms Reply To Email Address
If you have your Reply to email address set to an @yahoo.com or @aol.com address, emails sent by our system will be rejected. This includes new Submitted Form alerts, notifications to the submitter, and other alerts.
Problem: When our servers send an email related to an Online Form we use the Reply to email address as the source for the message. If the source email address is on the Yahoo or AOL domain the message is rejected due to Yahoo's security setting.
Resolution: Change the Online Form's Reply to email address to one that is not on the Yahoo or AOL domain. You can make this change by going to Features > Online Forms, click the Edit button next to the Online Form, and change the Reply to Email Address. Then click Save Online Form.