Struct for incoming messages.
@type t() :: %Inbox.Message{ attachments: [Inbox.Attachment.t()] | nil, from: [{String.t(), String.t()}], html: String.t() | nil, id: String.t(), raw: map(), subject: String.t() | nil, text: String.t() | nil, timestamp: DateTime.t() | nil, to: [{String.t(), String.t()}] | nil }