[dezren39]

Fixes #2637 (task 5421).

plain_address — the only address-shape gate for SmtpSink's sender and recipients — only checked that @ occurred somewhere, so @, a@, @b, a@@b.c and a@b@c.d all passed.

Change

• plain_address now splits on the first @ and requires: a separator exists, both local and domain sides are non-empty, and the domain contains no second @ — i.e. exactly one @ with non-empty sides. The existing ASCII-graphic / SMTP-delimiter character checks are unchanged.
• Doc comment records the intentional divergence from directory::normalize_email: no trim/lowercase, no dotted-domain requirement — this gate only shapes an address for the local relay.
• No call-site changes needed: SmtpSink::new already validates the configured sender through plain_address, and send already validates the recipient through it before TcpStream::connect_timeout.

Tests

• only_plain_ascii_addresses_pass_the_address_check extended: rejects @, a@, @b, a@@b.c, a@b@c.d; valid controls a@b.co and first.last+tag@example.test still pass.
• New a_malformed_sender_or_recipient_is_refused_before_connecting: malformed senders fail construction, malformed recipients fail send, and the test asserts no connection ever reached the relay listener.
• Verification via CI on this PR (CI-first; no local builds).

---

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• pkgs/id/src/directory_mail.rs

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Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-11-02T23:18:47.940147Z	849790c	PR opened

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