Prepare Exim host identity and SMTP acceptance policy

Exim combines SMTP transport with a programmable configuration language. Distribution packages may generate the effective configuration from split files, so determine which source files your installed package owns before editing a generated output.

Establish the receiving boundary

1. Install a maintained Exim build and follow the matching current specification.
2. Set the intended primary hostname and local-domain policy. Keep relay destinations and trusted submission clients separate from final local recipients.
3. Review SMTP ACLs, particularly recipient-stage decisions, before accepting public traffic.
4. Configure TLS certificates and an appropriate unprivileged delivery design; subscribe to distribution/upstream security updates.

Inspect the installed build and effective configuration using the documented command-line interface:

exim -bV

Review configuration output privately; do not publish expansion values or credentials while diagnosing a package mismatch.

Verify policy before moving MX

Test controlled local delivery, an unknown local recipient, and an unauthorized remote-to-remote relay attempt. Require the intended rejection at the correct SMTP stage rather than discovering the mistake after accepting a queue full of mail.

A valid SMTP listener does not create an IMAP mailbox or HTTP email API. Connect the intended local delivery/mailbox components explicitly. Preserve existing DNS and configuration for rollback until recipient policy, TLS, and queue handling have all been checked on the test domain.