The default for new installations of Notes and Domino is that email is not encrypted unless the user requests it for a specific message by clicking the Encrypt checkbox in the Delivery Options dialog.
See table four, here: https://www.ibm.com/support/knowledgecenter/SSKTWP_9.0.1/com.ibm.notes901.help.doc/mail_quickref_r.html?lang=en

Users can change this through a preference setting "Encrypt messages that I send" available in the Preferences dialog, under Mail - Sending and Receiving.
See here table 3, here: https://www.ibm.com/support/knowledgecenter/SSKTWP_9.0.1/com.ibm.notes901.help.doc/mail_quickref_r.html?lang=en

Administrators can control this setting centrally for all users if they want it to be in effect for all users.
See here: https://www.ibm.com/support/knowledgecenter/SSKTMJ_8.5.3/com.ibm.help.domino.admin85.doc/H_CREATING_A_DESKTOP_POLICY_STEPS.html

-rich

Rich, 

Thanks a lot for your reply. If the user choose to encrypt, do you know what is the key strength of the encryption key. Is it using the key that is in your notes id?

Thanks a lot for your help.

Kind Regards,