Mailing lists are subscriptions for receiving materials from inviduals or organizations, `kind 1923` can be used for subscribing to mailing lists owned by an user.
`tags` MUST contain an entry identifying the invidiual or organization subscribing to, in the form of `["p", "<pubkey, as a hex string>"]`.
The `content` MUST contain a valid e-mail address encrypted by same method of encrypted direct messages(NIP-04) for subscribed pubkey.