Q: Mail App doesn't show the mail headers

I am using the default presentation of mail message headers.

Suddenly, only the name of the sender appears, but not the To: and Cc: fields

(as I am on a French localized computer I don't know the words used in the English version, but this is something like Preferences:Presentation:Default Headers

When I hover the cursor over the two blank lines, mail addresses appear (but not names).

I had the issue one or two years ago and it disappeared without explanation.

I tried rebuilding the mailboxes, but no luck.

Any number of quit and relaunch did not help either.

I will start re-booting and report if it fixed the issue.

This is Apple Mail Version 7.3 (1878.6) under 10.9.5.

Best

Jean-François