Declares which mail servers may send on behalf of your domain.
Custom Domain Email Authentication
A practical hub for setting up branded sending domains on Resend and Brevo, and for verifying that your domain’s sending reputation stays healthy over time.
Why a verified custom domain matters
Sending from a domain you control — rather than a shared or generic address — gives mailbox providers clear signals about who is authorized to deliver mail on your behalf. This reduces spam-folder placement and strengthens long-term deliverability across marketing, transactional, and cold outreach flows.
Stronger brand presentation
A domain that sounds established can make outreach feel more professional than a freshly created sending identity. That matters in cold outreach, newsletters, lead generation, and transactional campaign environments where first impressions influence opens and replies.
Better technical trust signals
SPF, DKIM, and DMARC records tell inbox providers exactly which servers and signing keys are allowed to send for your domain, which is the single biggest factor separating inbox delivery from spam placement.
Cryptographically signs outgoing mail so it can’t be tampered with in transit.
Tells receiving servers what to do when SPF or DKIM checks fail.
Guides in this collection
- Resend domain verification — connect and verify a custom sending domain in Resend using SPF, DKIM, and bounce-handling DNS records.
- Brevo domain authentication — authenticate a custom sending domain in Brevo, including verification codes, DKIM, DMARC, and SPF merging.
- Domain reputation testing — check whether your sending domain or IP is listed with Spamhaus, and monitor inbox placement with Google Postmaster Tools.
The bigger picture: reputation over time
Authentication (SPF/DKIM/DMARC) proves who is sending; reputation is the ongoing trust score inbox providers assign based on complaint rates, bounce rates, and blocklist history. Even a perfectly authenticated domain can land in spam if its sending reputation degrades, which is why ongoing monitoring through tools like Spamhaus and Google Postmaster Tools is essential.