diff --git a/src/yunohost/certificate.py b/src/yunohost/certificate.py
index 7e6726f31..e240774e1 100644
--- a/src/yunohost/certificate.py
+++ b/src/yunohost/certificate.py
@@ -499,7 +499,7 @@ Subject: %s
     import smtplib
 
     smtp = smtplib.SMTP("localhost")
-    smtp.sendmail(from_, [to_], message)
+    smtp.sendmail(from_, [to_], message.encode('utf-8'))
     smtp.quit()
 
 
diff --git a/src/yunohost/diagnosis.py b/src/yunohost/diagnosis.py
index 29ffe686b..602efef8a 100644
--- a/src/yunohost/diagnosis.py
+++ b/src/yunohost/diagnosis.py
@@ -712,5 +712,5 @@ Subject: %s
     import smtplib
 
     smtp = smtplib.SMTP("localhost")
-    smtp.sendmail(from_, [to_], message)
+    smtp.sendmail(from_, [to_], message.encode('utf-8'))
     smtp.quit()