Revert "Remove ValidMailhostLayer to see if that fixes the deadlock on Jenkins."

This reverts commit 22d1c539e35b96a749227824f50c2718e75ed12f.
Revert "Revert "Remove ValidMailhostLayer to see if that fixes the deadlock on Jenkins.""

This reverts commit 7ecdcd2893644d4936c37fa6cf6d718dbe4d5dc3.
Fix the the_mail_setup_configured robot keyword.