We look into problems like this frequently. There are no technical issues that would prevent an AOL or Yahoo user from receiving a Hallmark ecard. It's possible that your recipient has set up their account to block email from unknown addresses. If the recipient has not authorized email from Hallmark.com, they may not receive their notification email telling them that you have sent them an ecard. If you think this may be the case, ask them to check their bulk mail or junk mail folder to see if the notification email was redirected there.
You can also try forwarding the confirmation email you received from us directly to your recipient. The confirmation email contains a link to the ecard you sent and instructions on how to pick up the ecard if they are unable to view it using the link.