feat: add reusable SMTP batch sessions

README.md

@@ -41,6 +41,18 @@ and requires explicit evidence-backed reconciliation before any deliberate
 resend. Business readers receive only counts and sanitized state; recipient
 refusal details require `mail:delivery:diagnostic`.
 
+Synchronous Campaign batches now preflight DNS, egress, connectivity, TLS, and
+authentication before the first message, then reuse the authorized SMTP
+connection for the bounded batch. A health check precedes reuse; a stale
+connection is reopened before the next message, while a connection loss after
+DATA starts remains outcome-unknown and is never replayed automatically.
+Systemic authentication, sender, and connectivity failures pause remaining
+Campaign jobs instead of producing one failure per recipient. Deployment
+operators can disable reuse or bound connection lifetime and reconnects with
+`GOVOPLAN_SMTP_BATCH_REUSE`, `GOVOPLAN_SMTP_BATCH_MAX_MESSAGES`,
+`GOVOPLAN_SMTP_BATCH_RECONNECT_ATTEMPTS`, and
+`GOVOPLAN_SMTP_BATCH_HEALTH_CHECK`.
+
 SMTP effects decrypt only SMTP credentials; Sent-folder effects decrypt only
 IMAP credentials. A connection loss after an effect starts is surfaced as an
 unknown outcome. Campaign does not automatically retry an unknown IMAP append,