Most cold email lands in spam for a few fixable reasons: missing or misaligned authentication (SPF, DKIM, DMARC), a new domain with no sending reputation, no one-click unsubscribe, high complaint rates, or spammy content and links. Google and Microsoft publish sender requirements for bulk mail. The Google sender guidelines say mail that does not meet them “might not be delivered as expected, or might be marked as spam”.
The causes split into reputation, authentication, compliance, and content. Reputation: a brand-new domain has no track record, so providers treat it cautiously and ramp trust slowly — sending high volume on day one looks like a spammer. Authentication: SPF, DKIM, and DMARC must all pass and align; a DKIM signature that doesn't match the From domain, or DMARC left at monitoring only, weakens trust. Compliance: bulk senders now need working one-click unsubscribe (RFC 8058) and must keep spam complaints low — Gmail's threshold guidance is to stay under 0.3%. Content: spammy phrasing, link shorteners, mismatched display names, and image-heavy mail with little text all push spam scoring up. Google's sender guidelines say mail that does not meet them “might not be delivered as expected, or might be marked as spam”. Fixing the root cause beats any attempt to mask it.
Authentication is the quickest of these to rule out: check your SPF record and check your DMARC record before you start rewriting content or blaming reputation.