SharePoint 2013: "The e-mail message cannot be sent. Make sure the e-mail has a valid recipient."

Recently, I ran into an issue in SharePoint 2013 in which a SharePoint workflow would not send email notifications to a few users selected via an InfoPath people picker control. The only thing that fixed it for me was the following.

1) Go directly to the All People list of the root site collection by using the following URL format.
http://[SiteCollectionURL]/_layouts/people.aspx?MembershipGroupId=0

2) Find the users experiencing the issue and remove them.

3) Add permissions to the site for each user again, as needed.

If this issue is occurring to many users throughout your entire SharePoint site collection hierarchy, you can apply the following farm wide fix:

1) stsadm.exe -o sync -deleteolddatabases 0

2) Manually run the User Profile to SharePoint Full Synchronization timer job. This job will take some time to run, but should fix the issue throughout the entire farm.