diff -r cc55093cc86d -r 1a5eb9a9f5b4 cubicweb/cwconfig.py
--- a/cubicweb/cwconfig.py	Thu Aug 01 09:20:40 2019 +0200
+++ b/cubicweb/cwconfig.py	Tue Aug 06 16:43:46 2019 +0200
@@ -1248,7 +1248,7 @@
                 return False
             for msg, recipients in msgs:
                 try:
-                    smtp.sendmail(fromaddr, recipients, msg.as_string())
+                    smtp.sendmail(fromaddr, recipients, msg.as_bytes())
                 except Exception as ex:
                     self.exception("error sending mail to %s (%s)",
                                    recipients, ex)