A different person created the mailing, so I checked, and that person’s Civi Contact was not linked to that persons Joomla user.
So I ran the “Synchronize Users to Contacts” to try and connect them. I found two Civi Contacts for the person. I deleted the one that wan’t connected to a Joomla user.
I asked the person to log back into joomla, delete the “draft mailing”, and to try and create a new one.
She was still getting the error. So she tried to do a different mailing based on a different group, and it worked fine.
So something is wrong with the smart group that she was trying to send the mailing to. I did a quick glance at the list of contacts for the failing smart group and some of the contacts are missing emails. Could that cause the error?
I believe that this person also originally created the smart group in question, so maybe her contact not being linked to a user, caused an issue when creating the smart group. Then when trying to mail to the smart group, caused the error.
Anyway, we should be good to go for now. If I find anything else out on why the group caused an error, I will post it.