Hallmark Card Studio E-mailed Cards Not Reaching Recipients
How do you know it was not delivered - i.e., did the sender receive a return email notice that it was undeliverable or did the recipient receive anything at all? If it was sent and just disappeared, there may have been a problem with the email address. If an "undeliverable" notice was returned to the sender, there may have been a problem with the recipient's server, email program or virus software.
Outlook has several settings which prevent certain types of emails from being delivered or received. It attempts to block anything that looks like spam. For example, emails with too many attachments, certain types of extensions, large picture files, emails being sent out to too many people at a time, etc. Some of these setting can be changed, but some cannot. You don't say which version of Outlook you are using. If it has not worked when you are sending the card directly from the Hallmark program using Outlook as your default email program, you might try opening Outlook and creating an email from there and attaching the card.
The source of the problem could also be the virus software of the sender or the receiver.