Sends mail for the email channel adapter over an SMTP relay.

Ported from LemonGateway.Transports.Email.Outbound. smtp_options/1 is the same function it always was — its configuration semantics are pinned by characterization tests that were written before the move and carried over unchanged, so a difference there is a regression rather than a design choice.

What the port had to change

The gateway version was handed a finished run (deliver(job, completed)) and read its reply headers out of job.meta.email_reply, which the inbound half had put there. A LemonChannels.Plugin adapter is handed a LemonChannels.OutboundPayload instead, which carries the recipient, the thread and the id being replied to — but deliberately not a Subject or a References chain, since neither means anything to a channel that is not email. Those come from LemonChannels.Adapters.Email.ThreadStore, which the inbound half populates; see its moduledoc for why that state is worth keeping.

Two behaviours did not survive, both because they described the old entrypoint rather than email:

• The job.meta guard that made deliver/2 a silent no-op for non-email runs. deliver/1 is only ever called for payloads addressed to this channel, so a payload it cannot send is now an {:error, reason} — an unsendable message should be reported, not swallowed.
• The "Attachment references" list appended to the body from the run's output files. In this pipeline a file is its own :file payload and is attached to its own message, so there is nothing to reference.