Direct mode — connects straight to the recipient's mail server via MX lookup. Any From address works. No account or API key needed.
Emails may land in spam (no SPF/DKIM) — which is fine for inbox filter testing. If you get a connection error, your ISP blocks port 25 (common on residential connections) — switch to Relay mode.
Note: All major relay services (Resend, Mailgun, SendGrid, Brevo) now require domain verification for the From address. Relay mode is useful if your ISP blocks port 25 — but you'll need a domain you control. For arbitrary From addresses, use Direct mode.