Email Deliverability Basics: Bounces, SPF, DMARC, Spam

A hard bounce is permanent: the address doesn't exist, the domain is wrong, or the mailbox is closed — keep sending to it and your sender reputation drops. A soft bounce is temporary: a full inbox, a rate limit, a momentary server error — a later retry can succeed. LDM's stop-list logic treats them differently: a hard bounce suppresses the address immediately, a soft bounce gets a bounded number of retries first.

RFC 7505 defines a null MX record ("0 .") that a domain publishes to declare it never accepts inbound mail — common for marketing-only or parked domains. If you're sending to a domain like that, every message hard-bounces by design; it isn't a problem with your sending setup, just suppress the address.

A managed host serving your site is a different system from whatever sends your mail — the host's own relay, a plugin such as WP Mail SMTP, or a separate ESP. Your domain's SPF record has to list every one of those services as authorized to send as you; miss one and mail from that path gets flagged or rejected on the receiving side.

Send a message to a set of seed inbox addresses and read the raw headers, or run a header analyzer — both show the SPF/DKIM/DMARC verdict the receiving server actually applied, which is closer to reality than a standalone DNS lookup.

It's rarely one cause — usually two or three at once: missing or misaligned SPF/DKIM/DMARC, a sending reputation that hasn't warmed up, and content patterns (links, images, wording) the filter has learned to distrust. Platform notification emails (Notion, 1C-Bitrix and similar) hit this constantly, because they're sent from shared infrastructure the recipient's filter doesn't recognize as belonging to you.

Trial-expiration emails get flagged easily — they look transactional but are usually sent in bulk from marketing tools with weak authentication. Fix the plumbing first (SPF/DKIM/DMARC on the sending domain, a warmed-up sender reputation), then keep the copy plain: one clear deadline, one action, no more than one link.

No-show follow-ups are exactly what spam filters watch for: sent right after an event, to a whole list, often with a recording link. Space the sends out, personalize the subject line, and verify placement on a seed test before a large send — especially the first time you run this sequence.