DKIM key length: 1024 vs 2048 — does it matter?

Short answer: use 2048-bit keys. 1024-bit RSA still validates at every major receiver in 2026, but it sits at the edge of factorability — well-resourced attackers can plausibly break a 1024-bit key, and a broken DKIM key means someone can sign mail as you that passes authentication.

The practical details:

• Deliverability difference today: minimal. Gmail and Microsoft don't visibly penalize 1024-bit signatures yet. The risk is security and future-proofing, not immediate spam-foldering.
• Why 1024 still exists: a 2048-bit public key doesn't fit in a single 255-character TXT string, and some old DNS providers can't split TXT values. Every mainstream DNS host (Cloudflare, Route53, Google Domains) handles this fine now.
• 512-bit keys are broken — factorable for a few dollars of compute. Gmail has rejected them since 2013. If you find one, rotate it today.
• Rotation: whatever the length, rotate keys periodically (and when staff with DNS access leave). Providers with automated rotation (SendGrid "Automated Security", M365 dual selectors) handle this for you — but only if their CNAMEs stay published.

Check what you're actually running: the DKIM checker reports approximate key length for any selector.