DNS and mail-flow diagnostics that tell you what to fix
Free checks for IT admins and MSPs. Each result explains what it means and the exact change to make, with no product to sell you.
Start from the symptom
All guidesMail is landing in spam
Check authentication, alignment and the sending IP's reputation.
Email Record LookupGuide: Why is my email going to spam? A diagnostic checklist for admins
Mail is bouncing or rejected
Check whether the sending IP is blocklisted or missing reverse DNS.
Email Blacklist CheckGuide: How to get off an email blocklist (DNSBL)
SPF says “too many DNS lookups”
Count the lookups and see which includes to flatten or drop.
SPF ValidationGuide: Fixing "SPF too many DNS lookups" (PermError)
DMARC fails even though SPF passes
Almost always an alignment problem between the From domain and SPF/DKIM.
Email Record LookupGuide: DMARC alignment, explained: why SPF passes but DMARC fails
I changed DNS and nothing happened
See what resolvers return now and how long the old TTL keeps it cached.
DNS Name LookupGuide: DNS propagation and TTLs: why your change hasn't shown up yet
Moving to a new DNS provider
Confirm the current nameservers and plan a cut-over without downtime.
Nameserver LookupGuide: Changing nameservers (DNS providers) without downtime
Who owns this IP address?
Find the network owner and the abuse contact to report to.
IP WhoisGuide: Reading IP Whois and RDAP results: owner, network and abuse contact
Why did this one message fail?
Paste its headers to see the path, delays and exactly which check failed.
Email Header AnalyzerGuide: How to read email headers: Received, Authentication-Results and DKIM
Is this host reachable from around the world?
Ping it from probes on four continents and see the round-trip time.
Ping CheckGuide: Ping, ICMP and what "unreachable" really means
Email delivery
View allEmail Blacklist Check
Is this mail server's IP blocklisted, and is its reverse DNS set up the way receivers expect?
Email Record Lookup
Is this domain's email set up correctly: MX, DMARC, DKIM and SPF?
Email Header Analyzer
Why did this message arrive late, land in spam, or fail authentication?
SPF Validation
Is this IP address, or this whole range, allowed to send mail for the domain?
Domains and DNS
View allWhois Search
Who is this domain registered with, when does it expire, and is it locked?
Nameserver Lookup
Which nameservers are authoritative for this domain, and which DNS provider runs them?
DNS Name Lookup
What does DNS return for this name right now?
DNS Propagation Checker
What does each public resolver return for this record right now?
DNSSEC Validator
Is this domain's DNS signed, and does the chain of trust validate?
CNAME Chain Resolver
Where does this CNAME chain end, and which record finally answers?
IPv6 Readiness Check
Can this domain and its mail servers be reached over IPv6?
IP addresses
View allWhat Is My IP?
Which public address does the internet see for me, and what does its reverse DNS say?
IP Whois
Who owns this IP address, and where do I report abuse from it?
Reverse DNS Lookup
Which hostname does this IP address claim, and does that name point back?
BGP Route Lookup
Which network announces this address block, and who is responsible for it?
Reachability
View allWeb and TLS
View allCalculators
View allLatest guides
Browse by topic- BIMI: requirements and setup for a logo in the inboxupdated 26 October 2026
What BIMI needs before an inbox shows your logo: DMARC enforcement, an SVG Tiny PS logo, a VMC or CMC certificate, and the DNS record that publishes it.
- Changing nameservers (DNS providers) without downtimeupdated 26 October 2026
How to move a domain to a new DNS provider without downtime: copy and verify the zone, handle DNSSEC, then change the delegation and wait out the caches.
- DKIM selectors: finding, verifying and rotating keysupdated 26 October 2026
What a DKIM selector is, how to find the one your mail platform uses, how to verify the published key, and how to rotate keys without failed signatures.
- DMARC alignment, explained: why SPF passes but DMARC failsupdated 26 October 2026
Which domains DMARC compares, relaxed vs strict alignment, and how to fix the most common "SPF pass, DMARC fail" and "DKIM pass, DMARC fail" cases.
- DMARC from p=none to p=reject without breaking mailupdated 26 October 2026
How to move DMARC from monitoring to enforcement: what to publish, which reports to read, how long each stage takes, and what to do with what they find.
- DNS propagation and TTLs: why your change hasn't shown up yetupdated 26 October 2026
DNS changes don't propagate; cached answers expire. How TTLs, negative caching and delegation caching decide how long an old answer survives.
Written and maintained by James Bayliss, an IT manager who has worked in IT support, systems administration, datacenter engineering and solutions architecture.