Mail client's configured to. Also, it establishes a history of key use in a public forum.

On 25 June 2014 10:20:50 GMT+01:00, stef <s@ctrlc.hu> wrote:
i noticed lots of users pgp-sign their mails to mailing-lists. what exactly is
the reason/usecase/attackvector you defend against for that? what exactly is
the reason for doing so on public mailing lists? and why does it make sense to
sign irrelevant messages like "+1" or "just kidding" - assuming no stego
usecase is in play.

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Sent from my Android device with K-9 Mail. Please excuse my brevity.