Troubleshooting
Decode the bounce. Fix the failure.
Gmail 550 5.7.26: why your mail bounced and how to fix it
Gmail now rejects bulk mail that isn't authenticated. What the 550 5.7.26 bounce actually means, the three things that cause it, and how to confirm you've fixed it.
550 5.7.1: email rejected per DMARC policy, and how to fix it
On Microsoft 365, a 550 5.7.1 rejection usually means your mail failed DMARC. Why it isn't always DMARC, the authenticated-but-unaligned trap, and the fix.
550 5.7.23: your mail failed SPF, and how to fix it
Microsoft's 550 5.7.23 and Gmail's 5.7.27 mean a receiver checked SPF and the sending IP wasn't authorized. The usual causes, including the High Risk Delivery Pool, and the fix.
SPF PermError: too many DNS lookups, and how to fix it
SPF allows only 10 DNS lookups. Cross the limit and receivers return permerror, which counts as not passing and can take DMARC with it. What counts, and how to come back under.
Reverse DNS and PTR records: why mail gets rejected
Strict receivers reject mail from IPs with missing or mismatched reverse DNS (Gmail 550 5.7.25). What forward-confirmed reverse DNS is, where the PTR record lives, and how to fix it.
Yahoo 554 5.7.9 and Gmail bulk-sender errors, decoded
Yahoo's 554 5.7.9 and Gmail's 4.7.x and 5.7.x bulk-sender codes, decoded. Which mean fix your authentication, and which mean slow down and fix reputation.
DKIM failed (dkim=fail): why your signature did not verify
A dkim=fail means your DKIM signature did not verify. The handful of things that cause it (a modified body is the big one), and how to fix each.
SMTP error codes: how to read an email bounce
Every bounce carries an SMTP reply code and an enhanced status code. How to read them, the rule that says retry or fix, and what the common authentication codes mean.
SPF ~all vs -all: softfail, hardfail, and which to use
The qualifier at the end of your SPF record decides what receivers do with unlisted senders. The difference, what receivers actually do, and why DMARC changes the choice.
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