ProofpointPOD - Multiple protected emails to unknown recipient

Original query, unchanged.

let lbtime = 30m;
let lbperiod = 14d;
let knownrecipients = ProofpointPOD
| where TimeGenerated > ago(lbperiod)
| where EventType == 'message'
| where NetworkDirection == 'outbound'
| where SrcUserUpn != ''
| where array_length(todynamic(DstUserUpn)) == 1
| summarize recipients = make_set(tostring(todynamic(DstUserUpn)[0])) by SrcUserUpn
| extend commcol = SrcUserUpn;
ProofpointPOD
| where TimeGenerated between (ago(lbtime) .. now())
| where EventType == 'message'
| where NetworkDirection == 'outbound'
| extend isProtected = todynamic(MsgParts)[0]['isProtected']
| extend mimePgp = todynamic(MsgParts)[0]['detectedMime']
| where isProtected == 'true' or mimePgp == 'application/pgp-encrypted'
| extend DstUserMail = tostring(todynamic(DstUserUpn)[0])
| extend commcol = tostring(todynamic(DstUserUpn)[0])
| join knownrecipients on commcol
| where recipients !contains DstUserMail
| project SrcUserUpn, DstUserMail
| extend AccountCustomEntity = SrcUserUpn