Your domain has no reputation yet. That is the whole answer.

Spam filtering happens before content analysis at most receivers: the sending domain and IP are scored first, and a cold domain scores poorly regardless of how legitimate the message is. That is why a brand-new SaaS sees its first password-reset email land in spam while the identical email works fine from a competitor’s year-old domain.

The corollary matters more than the fix: nothing you write in the email compensates for missing reputation. Content polish is the last mile, not the first.

Before reputation can accumulate, the domain needs its identity records in place: SPF authorizing the sending infrastructure, DKIM signing that survives forwarding, and DMARC telling receivers what to do when alignment fails. For transactional mail, send from a dedicated subdomain (mail.yourapp.com) so a problem in one stream cannot tax the reputation of the corporate apex domain.

This is why Marmunk requires domain verification with these records before any send is approved: a send from an unverified domain is a send from a domain with no identity, which is exactly the profile of a spammer.

Reputation builds from observed behavior: low, steady volume; low bounce and complaint rates; and engagement from recipients. The pattern receivers reward is a slow ramp — small daily volume that grows as bounces stay negligible. The pattern receivers punish is a cold domain suddenly emitting its full backlog.

AI agents need this called out: an agent that imports a contact list and “helpfully” announces a product to all of them at once will torch the domain’s reputation on day one. Hard sending envelopes — daily and monthly recipient caps that the agent cannot raise — exist partly for exactly this scenario.

The good news: transactional email is the easiest case. Recipients asked for the mail (they created the account, placed the order), mailboxes weight that engagement heavily, and volumes are user-driven rather than campaign-driven. A transactional sender that verifies its domain, ramps honestly, and keeps bounces near zero typically builds working reputation in weeks, not months.

Set up and verify your sending domain with the domain onboarding flow, and watch per-message evidence — submission, delivery, bounces — through the send-debugging guide while reputation accumulates.