As a standard mail protocol that defines mailers that run over the Internet, SMTP is not an end-to-end mechanism. Because of this protocol limitation, TLS security through SMTP does not include mail user agents. Mail user agents act as an interface between users and a mail transfer agent such as sendmail.
Also, mail might be routed through multiple servers. For complete SMTP security the entire chain of SMTP connections must have TLS support.
Finally, the level of negotiated authentication and privacy between each pair of servers or a client and server pair must be considered. For more information, see Authentication Services in System Administration Guide: Security Services.