Represents a token that encapsulates the information necessary for callbacks from
the EmailProvider's asynchronous Send methods.
CMS.EmailEngine (in CMS.EmailEngine.dll) Version: 9.0.0
public sealed class EmailToken
The token contains the information about site, timestamp of the last send attempt,
the message itself and related object in the DB as well as the used ID (for mass messages).
This is required in order to persist the message in case of delivery failure or when
attempting to resend the message. Additionally, an SMTP server is present in the token as
well for the correct function and signaling in the e-mail queue (idle/busy notifications
A unique identifier was added to the token to distinguish the threads in the callback so
that only threads that own the queue item will process it.