MXToolbox

mxtoolbox.com

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A suite of free network diagnostic tools focused on email infrastructure — MX record lookup, SPF/DKIM/DMARC testing, blacklist checking, and SMTP diagnostics.

Why it is useful

The go-to diagnostic toolkit for troubleshooting email deliverability problems — checking whether a domain is blacklisted, validating SPF and DMARC records, and testing SMTP connectivity are all one click away. The blacklist checker alone is worth bookmarking, as it simultaneously queries over 100 blacklists. Anyone managing email infrastructure uses this regularly.