1. Authentication: SPF, DKIM and DMARC
The three standards prove that a message comes from your domain and arrived unchanged. Gmail, Yahoo and Outlook expect them from anyone who sends larger volumes.
- SPF lists the servers that may send for your domain.
- DKIM adds a cryptographic signature to every message.
- DMARC tells receivers what to do when a check fails and sends you reports.
2. Infrastructure: reverse DNS, HELO, TLS and IP reputation
The sending server needs a matching PTR record, a HELO name that belongs to its IP and an encrypted connection through STARTTLS. If the IP sits on a blacklist, even the best content helps little.
3. Content and format
Spam filters score structure and text. A clean plain-text part next to the HTML, a meaningful subject, a balanced ratio of text to images, spelled-out links instead of short URLs and, for newsletters, a List-Unsubscribe header with one-click unsubscribe all help.
How to proceed
- Send a test message to the address sender.report shows you.
- Open the report and work through the findings by importance: errors first, then warnings.
- After you change something in DNS, click “Re-query DNS & re-score” in the report. You do not need to send the test mail again.
- Repeat the test until no errors remain and the score is in the green range.