Track SMTP submission robustness improvements

[bbottema]

SMTP callers need to know what the server accepted, when a retry could duplicate mail, and what happened when a connection failed. This tracks the independently reviewable improvements in 03_SMTP_ROBUSTNESS_IMPROVEMENT_PLAN.

The project builds on the 10.0 submission receipts (#710), terminal observer (#712), and exact EML support (#713). It does not add automatic retries, a durable spool, or mailbox-delivery guarantees.

Phases 1 and 2 are scheduled for implementation now. Step 3 (#726) has reached its provider safety gate and is blocked pending safe Angus/pool interruption hooks; the queue controls in #725 can be reviewed independently. Later steps remain proposals; they can ship in separate releases.

Step	Child issue	Planned release	Shipped release	User-visible result
1	#723	10.0.0	—	Per-recipient replies and conservative retry guidance
2	#725	10.0.0	—	Bounded async demand and visible overflow
3	#726	10.0.0	—	Deadlines and protocol-aware cancellation
4	Not created	Unscheduled	—	SMTP capability diagnostics
5	Not created	Unscheduled	—	Explicit authentication-over-TLS policy
6	Not created	Unscheduled	—	Complete DSN envelope metadata
7	Not created	Unscheduled	—	Per-message REQUIRETLS
8	Not created	Unscheduled	—	SMTPUTF8 and 8BITMIME requirements
9	Not created	Unscheduled	—	SIZE against finalized transmitted bytes
10	Not created	Unscheduled	—	Reproducible conformance and fault-injection tests

Each scheduled step gets its own linked sub-issue, exact release milestone, tests, documentation, and release entry. Keep this parent open until the steps are completed or explicitly moved to a successor plan. PIPELINING/CHUNKING remain parked under #699. Benchmarking is outside the current work.

[bbottema]

The supporting-library part of #726 shipped on 8 September 2026: Generic Object Pool 2.5.0, Clustered Object Pool 4.1.0, and SMTP Connection Pool 4.1.0.

The release narrative is stopping abandoned jobs while they wait for pooled resources, with opt-in controls, one acquisition budget, safe ownership handoff and observable cleanup. SMTP additionally offers an explicit capability for providers that can safely abort their own transport.

This is supporting infrastructure, not yet a Simple Java Mail send-cancellation feature: downstream integration remains under #726, and built-in Angus physical abort is still unsupported. These additions are separate from the earlier waiter-wake-up patch releases.