If you are unable to connect and get this error:
# ssh -v firstname.lastname@example.org ERROR: ssh_exchange_identification: Connection closed by remote host
To get more info about error you might try to add verbose command:
# ssh -v email@example.com
Check if you user has shell assigned:
# cat /etc/passwd |grep jenkins
jenkins:x:1002: 1002 :Jenkins Automation Server:/var/lib/jenkins:/bin/false
We can see shell is not allowed so change it:
# chsh jenkins
Changing shell for jenkins.
New shell [/bin/bash]: /bin/sh
Now create keys with ssh-keygen and copy them with ssh-copy-id command.