From the FAQ
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Green or red 'lock' icon when viewing a message, DKIM and SPF
First, this is new in 1.6.1 and can be turned off in app settings -> message view -> “Show security info”.
What it is:
DKIM and SPF are technologies related to email transmission security.
A green lock tells you that the message you’re viewing was “signed” by the mail server on the sending side and the signature “passed verification” by the mail server on the receiving side (the one hosting your mail account).
In practical terms, it means that the message’s content has not changed while en route.
A red lock tells you that either DKIM or SPF “failed to validate”, so either the message’s content, as received, is not what was sent, or the sending party used a computer which is not “authorized” for sending messages from his/her email’s domain (e.g. a message ostensibly from “billg@microsoft.com” but coming from a computer that has nothing to do with Microsoft).
Even if you get a red lock, it does not necessarily mean that someone’s intentionally tampering those messages with malicious intent: the indicator is purely technical, it just means that your (receiving) mail server decided that the digital signature didn’t validate, could be due to a software bug on the sending or the receiving mail server(s).
