Email authentication is the set of DNS records (SPF, DKIM, and DMARC) that prove your emails genuinely come from your domain. Without them, cold outbound is far more likely to be filtered or blocked outright.
The three records
SPF lists the servers allowed to send for your domain. DKIM adds a cryptographic signature that proves the message was not altered and came from you. DMARC tells receivers what to do when SPF or DKIM fails, and gives you reporting. Together they are table stakes for reaching the inbox.
Why it is non-negotiable now
Major mailbox providers have made authentication a requirement for bulk senders, not a nice-to-have. Missing or misconfigured records tank deliverability no matter how good your warmup or copy is. It is the first thing to fix before any cold campaign goes out.