Guides & answers

Everything we publish about keeping outbound email deliverable, grouped by the problem you have right now. Every page is written from what the DNS and blocklists actually return, and the measured pages are re-checked on every revision.

Blacklisted — get delisted

• Email blacklist removal: every major list, how to delist, how long it takes — the master guide; start here if you don't know which list.
• Spamhaus removal (ZEN, SBL, CSS, XBL, PBL, DBL) — return-code table, bounce decoder, per-list walkthroughs.
• Barracuda blacklist removal (BRBL) — the corporate-inbox list; what to write in the removal form.
• SpamCop removal — auto-expiring; find the report source.
• Why does my Spamhaus CSS listing keep coming back?
• Is UCEPROTECT Level 3 safe to ignore?
• Blacklist checker — 10 lists, no signup.

Bounce and error codes explained

• Microsoft 365: 451 4.7.500 Server busy (S77714 / S77717 / S77719) — you or Microsoft?
• Microsoft: 550 5.7.708 Service unavailable, access denied, traffic not accepted from this IP
• Gmail: 421 4.7.28 unusual rate of unsolicited mail
• Gmail: 550 5.7.1 message likely suspicious / blocked
• Why Microsoft is blocking cold email in 2026, and what still gets through

SPF, DKIM and DMARC — setup and measured data

• SPF DNS lookup cost of every major provider's include — measured — 33 providers counted by our resolver; Google and Microsoft now cost 1, not 4.
• Google Workspace DKIM & SPF setup · Microsoft 365 · Zoho · SendGrid · Mailchimp
• DKIM 1024 vs 2048: does key length matter?
• DMARCbis (RFC 9989): what changed, which tags are deprecated
• Checkers: SPF (with lookup counter) · DKIM · DMARC · DMARC report analyzer · Domain grader

Provider and policy changes that affect senders

• Microsoft SNDS removed spam-trap data — what to monitor instead
• Google Postmaster Tools v2: reputation scores are gone — what replaces them
• Does the EU AI Act require labelling AI-written cold email? · Article 50 disclosure, in detail · AI SDR compliance checklist 2026

For developers and agents

• Free JSON API — every checker, no key required at free-tier rates.
• SPF include-cost dataset (JSON, CC BY 4.0)
• llms.txt

About the people and method behind these pages: About Aurelius SDR.