In transit where ? When a message is sent from PM to any other service it must rely on that service ' s ability to offer TLS for encrypting the connection . If the remote server doesn ' t support it , PM is forced to transmit the email in plain text . Anyone in the middle of the connection can see the email . If your mail is encrypted with a key , then the plaintext email will be sent , but the message will be encrypted headers will be exposed . If you do not have encryption on the message , the message will be sitting in the recipients inbox , unencrypted for anyone to read whom has access . If the receiving server has TLS , the plain text email will be sent in an encrypted TLS tunnel only to be made available in plain text in the inbox of the user . No one will see the message in transit . If the message is encrypted with a key , the plain text of the message is encrypted and then transmitted via a TLS tunnel . Only the recipient with the matching key can view the letter regardless of who is in the middle or able to view the inbox . This is assuming no successful MITM is in effect against the server and PM . If the recipient is another PM user , it should be 100 % encrypted from send to receive . TLDR If PGPGPGSMIME is an option for encryption , use it . You never know what the receiving end is capable of .