Email infrastructure and deliverability tools
Work out what a bounce is telling you, audit a domain's authentication, or find the mechanism that quietly switched off an SPF record. Seven tools, built and maintained by Rastu Singh, an infrastructure engineer working on high-volume email. They run in your browser: nothing you paste is sent anywhere, and there is no account to make.
Domain check
Is this domain's email authentication set up correctly?
OpenBounce classifier
What is this bounce or deferral telling me?
OpenWarm-up planner
How fast can I ramp this domain or address without getting filtered?
OpenBlocklist check
Is this address or domain listed, and is the list still working?
OpenDMARC report reader
Who is sending mail as my domain, and what is failing?
OpenHeader analyser
What went wrong with this message, and whose problem is it?
OpenSPF lookup counter
How close is this SPF record to the ten-lookup limit?
OpenOne message, all the way through
A real swaks run against Gmail's MX, captured with --quit-after RCPT so
the handshake completes and no message is ever sent. Every line is a place delivery can
fail, and most of the work is knowing which one it failed at.
Read it annotated, line by line →
SMTP response reference
A 550 tells you the delivery failed and nothing else. The enhanced code beside it says which subsystem refused and whether anything you change will alter the answer, which is the whole difference between retrying and suppressing.
58.7% publish DMARC. 20.9% reach p=reject.
Original measurement across 100,000 domains from the Tranco list. Of those that publish DMARC at all, 35.8% leave it at p=none where it blocks nothing, and 20.8% enforce with no rua address, so they cannot see what they are enforcing. Methodology and the raw dataset are published alongside the findings.
doi:10.5281/zenodo.22832936 · CC BY 4.0