Engineering
Deliverability guides and product notes — no filler.
Why regex email validation fails
Regex validates syntax, not deliverability — valid-looking addresses still bounce. Why regex email validation fails, and the layered check that works.
How to validate an email address in Rails
Validate an email address in Rails in three layers: a URI::MailTo format rule, a Resolv MX validator, and an SMTP mailbox check via API.
How to validate an email address in Express
Validate an email address in Express: express-validator for syntax, a dns/promises MX lookup, and a verification API for the mailbox.
How to validate an email address in Flask
Validate an email address in Flask properly: WTForms' Email() for syntax, an MX lookup via check_deliverability, then an API for the mailbox check.
How to validate an email address in Spring Boot
Validate an email address in Spring Boot: @Email for format, a JNDI MX lookup, and an API-backed ConstraintValidator — with working code.
Validate an email address in ASP.NET Core
Validate an email address in ASP.NET Core in three layers: DataAnnotations for syntax, DnsClient.NET for MX, and an async FluentValidation API check.
Validate an email address in Next.js
Validate an email address in Next.js with a shared zod schema, a DNS/MX check, and a server-side API mailbox probe in a Route Handler.
SMTP response codes explained for email senders
A field guide to the SMTP response codes email verifiers read: 2xx, 4xx, and 5xx, what each means for deliverability, and the verdict each one produces.
How email verification works: the 8 stages
A stage-by-stage look at how email verification works, from syntax to the SMTP mailbox probe — what each check can prove, and what it can't.