An EmailSubmission create's response carries inbuxa:held (dlp-and-mail-flow-rules spec, §2.6, §4): true when the message is held for review, false otherwise, so the webmail can say so at once. A sender can't read the review queue, and a held message's sendAt is its real send time, not the century-off release, so this is how the sender learns. mail_rules_tests checks both values.