If you use postfix with DKIM milter and start getting such error:
no key selected for signing milter-reject: END-OF-MESSAGE from localhost
make sure you have correct version of milter_protocol in your main.cf:
/etc/postfix/main.cf: # Postfix ≥ 2.6 milter_protocol = 6 # 2.3 ≤ Postfix ≤ 2.5 milter_protocol = 2