From that article: Unfortunately, the SMTPS port is only used for the submission of authenticated mail by mail clients. Opportunistic encryption between SMTP s

From that article: Unfortunately, the SMTPS port is only used for the submission of authenticated mail by mail clients. Opportunistic encryption between SMTP servers, which is extremely important for preventing passive eavesdropping of email, requires STARTTLS on port 25 So for the sort of thing that Google are talking about STARTTLS is the only option. Some of the disadvantages aren't such a big deal for large-scale server-to-server transport. But protocol downgrade attacks should certainly be a worry.