6 min read
Cleaning a newsletter list before you hit send
What to remove from a newsletter list before a send: disposable and dead domains, provider typos, system addresses and sometimes role addresses. With a script.
Email validation
Syntax, DNS, MX, SMTP and normalization: what each check catches and what it misses.
- 7 min read
Building a simple email risk score from free signals
A practical email risk score for signups: which free signals to use, hard rules vs points, score bands, and how to calibrate it against your own abuse data.
- 6 min read
Signup domains with no website: what it tells you
A signup domain that receives mail but has no website is a useful fraud signal, not a verdict. What DNS shows, the innocent cases, and how to score it.
- 5 min read
Catch-all email domains and what they mean for validation
A catch-all domain accepts mail for any address, so verifiers can't confirm a mailbox. How detection works, why it's common in B2B, and how to treat it.
- 6 min read
Gmail dots, plus addressing and duplicate accounts
Why jane.doe+trial2@gmail.com and janedoe@googlemail.com are one inbox, and how to build a canonical email key that catches duplicates without breaking login.
- 7 min read
SMTP mailbox verification: why it's unreliable
What an SMTP mailbox probe really tests, the six ways its answers mislead you (VRFY, catch-all, greylisting, port 25 blocks), and what to use instead.
- 6 min read
Internationalized email domains: punycode and normalization
Handle addresses like jane@bücher.de: convert the domain to punycode with UTS 46, leave the local part alone, and avoid the traps that split one domain in two.
- 7 min read
DNS-over-HTTPS for domain checks in serverless code
Run MX, A and AAAA lookups from edge functions and serverless runtimes with the Cloudflare and Google DoH JSON APIs: request format, status codes and gotchas.
- 6 min read
Checking MX records to see if a domain can receive email
How to tell from DNS whether a domain accepts mail: MX lookups, the implicit MX fallback, null MX, and which DNS errors should never block a signup.
- 6 min read
Why an email regex is never enough
A regex can only tell you an email address is shaped right. What to match, what to deliberately reject, and the checks a pattern can never replace.
- 6 min read
Email validation vs verification: what each one catches
Validation checks that an address could work. Verification proves someone receives mail there. What each layer catches, misses, and where it belongs.
- 7 min read
What SPF and DMARC records tell you about a signup domain
SPF and DMARC show whether a domain's owner set up outbound mail authentication. How to look them up over DNS, what they prove, and what they don't.
- 6 min read
RDAP vs WHOIS: looking up domain registration data
RDAP replaced WHOIS for generic TLDs in 2025. How the two protocols differ, how to query RDAP in code, and where you still need WHOIS for country-code domains.
- 6 min read
Email verification services vs domain classification APIs: which do you need?
Verification services check if a mailbox will accept mail. Classification APIs tell you what kind of address it is. Which job needs which, and when to use both.
- 6 min read
Free email validation APIs compared (and what "free" really includes)
Free tiers of ZeroBounce, Abstract, Hunter, MailboxValidator, Emailable, Kickbox and pingutil compared: quotas, resets, SMTP checks and where each fits.