The standalone thread-pool fallback in _deliver_result() runs inside the
`except RuntimeError:` block (taken when asyncio.run() sees a running loop).
When future.result() raised there (SMTP ConnectionError, timeout, etc.), the
exception was NOT caught by the sibling `except Exception:` — it escaped
_deliver_result() and crashed the whole delivery loop, silently skipping every
remaining target. Multi-target delivery (e.g. deliver: 'email:a,email:b') is a
documented feature, so this broke a promised contract.
Wrap the fallback in its own try/except so a per-target failure is logged with
exc_info and the loop continues to the next target.
Fixes#47163
242c9639a8