Hi @Maria_pl ,
Does the send-mail processor produce any errors when the message fails to send? Also, please check the NiFi logs (nifi-app.log should be the one that might give you a hint as to what is happening in the background, but while there you might want to check the rest of the NiFi logs).
I see that you are using the latest version of NiFi. We are currently on version 1.10.0 and PutEmail works fine.