Yahoo! has implemented a change in policy to request "reject" for any email sent with a "From" attribute of that does not originate from Yahoo! servers.
What does this mean for Luminate Online?
The way that Luminate Online creates email when your constituents send email is to use the constituent's email address for the "From" attribute.  Constituent sent email originates from the Participant Center in TeamRaiser (PC classic and PC2), TellAFriend, eCards and Advocacy submissions that deliver to representatives via email.  This ensures that the recipient realizes that the communication is from the person that they know and that it is not unsolicited.  Your organization domain is a part of the delivery listed as the "Sender", which is why for some email clients (like Outlook) you will see emails delivered as "From Org Name on behalf of Bill Smith."
This is a feature designed to work a similar way that many other softwares use for peer to peer email.  This does mean that the change Yahoo! has made does affect email that constituents (end users) can send.   This means that delivery of these emails for constituents who have an email address on their constituent record will have poor deliverability to any email service provider that uses the DMARC protocol. 
Yahoo!'s help article on DMARC

This was also found to affect Teamraiser coaching emails if the configured sender email address was from a domain that implemented a "reject" policy via DMARC.  This was especially an issue for those clients who allowed sending coaching emails via the event management center as the event manager would be more likely to use a personal email address which might be from an affected domain.
What is Blackbaud doing to address this?
As you may expect this has caused a large response from many email sending organizations and corporations.
Luminate Online will be implementing a change in the functionality of constituent originated email (Participant Center, TellAFriend, ECard)  to allow for the best deliverability of these messages.   The "From" email address will become the site "From" address that is set in the site settings.  This does not mean that replies to constituent emails will go to the site address, they will still go to the participant's email address (the Reply-to address).  
We are making this change to ensure that we deliver email in a format that has both the best possible deliverability and identifying information for the constituent that sent the email so that the recipient recognizes the sender.

*UPDATE* 14.4.3 emergency update was installed for North American clients 4/21/2014 with this change to the product.  See the bottom of the article for a link to screenshots showing the format of the constituent sent emails.

For coaching emails, an interim change was made in Luminate Online release14.7.0a to warn event managers to not use email addresses from the domains affected.

In Luminate Online release 14.8.0, coaching emails no longer offer the ability to customize the email address used for the From attribute.  Instead, as with PC2 sent email, the From address is taken from the site option NOTIF_FROM.

What do you need to do?

The email address used in the FROM section of the constituent sent email will be the "NOTIF_FROM" email address on your site (under the Setup menu, Site Options)

Check to be sure this is an email address that represents your organization, and also you can verify that the domain on that email address has SPF configured.  To check that, please view this article.

Click here to see screenshots of the various large email providers and how email will appear there.