@startuml
skinparam NoteTextAlignment left
skinparam Padding 2
skinparam ParticipantPadding 40
skinparam LifelineStrategy solid
autonumber

participant "FHIR Server\nREST API"     as S
participant "Client"                    as C
participant "Email Server\n(SMTP/S, Direct)" as E


group Negotiating a Subscription
  C->S: Create Subscription\n""channel.type: email""
else Without handshake
  S-->C: Success\n""Subscription.status: active""
  |||
else With handshake
  S-->C: Success\n""Subscription.status: requested""

  S->E: Message with verification, opt-in, opt-out, etc.
  return SMTP Success, e.g.:\n""250: OK""

  ...

  note right of S
    Server updates Subscription:
    ""Subscription.status: active""
  end note

end

group Email Subscription Processing
else Server sends heartbeat
    S->E: Message with readable content and attached Bundle:\n""            Bundle.type: history""\n""SubscriptionStatus.type: heartbeat""
    ...
else Server sends event-notification
    S->E: Message with readable content and attached Bundle:\n""            Bundle.type: history""\n""SubscriptionStatus.type: event-notification""
    ...
end

@enduml