I use emailext in a multibranch pipeline with 

recipientProviders:[culprits()]

and also with

recipientProviders: [developers()]

Both scenarios are failing.
 
I get 

Not sending mail to user user.name@company.com with no permission to view 001_sandbox  » multibranch test » master #34An attempt to send an e-mail to empty list of recipients, ignored.

The correct user ("user.name@company.com") is recognized.  As the log suggests, the user has no permission to view the Jenkins job, but that's not true. This user is able to see (and run) the multibranch pipeline in Jenkins UI.

I use Atlassian Crowd as user management system and enabled project-based security.

In https://jenkins.mycompany.com/user/username/configure I can see the correct email address ("user.name@company.com") is set for that user.