Spotlight does not index mail Spotlight does not deliver any result from mail messages (although searching inside Mail.app works)

I tried to re-index the mails with the following result:

$ mdimport -Vp -d 4 -r /System/Library/Spotlight/Mail.mdimporter
Total processing time spent in importer plug-ins: 0.000000 seconds for 0 files
Top 0 most expensive files for importer plug-ins:


What could be wrong?
You can go for a re-index of the whole drive. Check out this help page at Apple: Re-index folder or drive

Also looking in there might show that the mail folder has been hidden for privacy.