Security instinct says a bigger key must be better. For DKIM on transactional and marketing mail, the practical answer is that 1024-bit keys are the right choice—and no inbox provider rewards you for going larger.
1024-bit is compliant and universally accepted
RFC 8301 §3.2 establishes 1024 bits as the minimum length verifiers must support and recommends signers use at least that. Gmail, Outlook, Yahoo, and Apple all accept 1024-bit signatures, and 1024-bit keys remain widely used across the industry. This is why Resend signs outbound mail with 1024-bit keys.
The bulk sender rules don’t care about key length
The bulk sender requirements from Google, Yahoo, and Microsoft—the ones that apply once you send 5,000+ messages a day—require DKIM alongside SPF and DMARC. None of them mandate a key length. A 1024-bit signature satisfies them completely.
What 2048 actually costs you
- Latency at scale. Larger keys take longer to sign and verify on every single message.
- DNS friction. The longer public key doesn’t fit comfortably in a TXT record. Some DNS providers hit character limits or force you to split the value across strings—a classic source of broken DKIM records and failed verification.
- Zero placement upside. Mailbox providers do not give 2048-bit signatures better treatment. A valid 1024-bit signature and a valid 2048-bit signature are, to the inbox, the same thing: valid.
The exception
If your security or compliance team has a hard policy requiring 2048-bit keys, that’s a policy decision—a legitimate one—but it’s not a deliverability decision. Make it with eyes open about the DNS setup it demands, and test the record carefully once it’s live.