Email encryption in transit (TLS)
Gmail supports encryption in transit using Transport Layer Security (TLS), and will automatically encrypt your incoming and outgoing emails if it can. Some other email services don't support TLS, and therefore messages exchanged with these services will not be TLS encrypted.
In Gmail on your computer, you can check that a message you’ve received was sent over TLS by clicking the small down arrow at the top-left of the email and reading the message details.
It is understood that Google is likely giving some preferential deliverability scoring to emails sent through encryption.
Good News. Marketo implemented Opportunistic TLS in the middle of 2015 so we are ahead of the ball!
Example of mail sent without encryption
Example of mail sent with encryption
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