What is the purpose of Cisco Email Encryption on Cisco ESA?
to ensure anonymity between a recipient and MTA
to ensure integrity between a sender and MTA
to authenticate direct communication between a sender and Cisco ESA
to ensure privacy between Cisco ESA and MTA
Overview of Encrypting Communication with Other MTAs:
AsyncOS supports the STARTTLS extension to SMTP (Secure SMTP over TLS).
The TLS implementation in AsyncOS provides privacy through encryption.
https://www.cisco.com/c/en/us/td/docs/security/esa/esa14-0/user_guide/b_ESA_Admin_Guide_14-0/b_ESA_Admin_Guide_12_1_chapter_011001.html?bookSearch=true
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