SentMail
    sentAt       UTCTime
    sentBy       InstanceId
    objectId     MailObjectId Maybe
    bounceSecret BounceSecret Maybe
    recipient    UserId Maybe OnDeleteCascade
    headers      MailHeaders
    contentRef   SentMailContentId
    deriving Generic

SentMailContent
    hash    MailContentReference
    content MailContent
    Primary hash
    deriving Generic