Missing /var/log/mail.log (after deleting it) - How do I recreate it?

Solution 1:

1. Remove the files belonging to root (If there), as suggested by @Dan.

2. Restart MTA(sendmail in my case) and syslog, as suggested by @drookie

   sudo service sendmail restart 
   sudo service rsyslog restart
   

Bonus info: The files will then be created as syslog:adm

Solution 2:

touch /var/log/mail.log /var/log/mail.err, then restart whatever is writing to them, like your MTA or syslog.