Please try the following steps until the issue is resolved:

Check the Email Mapping settings in Online Express
1. Select Online Express on the Navigation bar
2. Click the Account menu and select Integration options
3. Highlight Email mapping
4. Review the Email mapping options to make sure that the Email types selected in this window are set to look for the same email type that's been set on the constituent records that have been included in the email list

If using a newly created Email list, wait a moment for the list to refresh in Online Express. It can take up to 10 minutes for the list to refresh in Online Express with the new email information, so if the email list was just created and then quickly used to send a test email, this message may appear because Online Express hasn't refreshed the list yet. Some lists, especially very large ones, can take time to fully upload.