Deliverability is decided before the subject line. Mailbox providers check whether the sending domain has told them who is allowed to send on its behalf, and a domain that has not is treated with suspicion by default.
The three records
SPF lists the servers permitted to send for the domain. DKIM signs each message so it can be checked for tampering. DMARC tells providers what to do when the other two fail, and sends you a report. All three are DNS records your email platform will give you the values for.
Then protect the reputation
- Send from a subdomain for marketing so a bad campaign cannot hurt the main domain.
- Remove addresses that have not opened in six months; they drag the rest down.
- Warm up a new domain with small volumes for two weeks before the big send.
