Most deliverability failures come down to missing authentication records, a sending domain with no prior history, or a list carrying too many invalid addresses. Authentication means having SPF, DKIM, and DMARC records configured for your sending domain. These tell receiving servers that your emails are legitimate and significantly reduce the chance of being flagged. If you're sending cold outreach from a domain that's missing any of these, inbox placement will suffer regardless of how good the copy is.
Domain warming matters for new or low-volume sending domains. Starting with large send volumes from a domain that has no prior history triggers spam filters. Ramp up gradually, beginning with 20-30 emails per day on a new domain, increasing slowly over several weeks, and monitoring bounce and complaint rates as you go. High bounce rates accelerate reputation damage, so verifying your list before sending is as important as authentication setup. A domain that bounces more than 3-5% of its sends will struggle to reach inboxes regardless of warming.