Deliverability hub

Email deliverability, fixed

Why your mail bounces, gets deferred, or lands in spam — and the exact steps to get it back in the inbox. Start with the fix for the problem you have, or test your domain in 30 seconds.

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Deliverability is whether your email reaches the inbox — not just whether the server accepts it. Every delivery problem traces back to one of three things: authentication, reputation, or engagement. Get all three right and your mail lands; break any one and you get a bounce, a deferral, or the spam folder.

Authentication proves who sent the message. Publish SPF, sign with DKIM, and set a DMARC policy so the passing domain aligns with your visible From address. Since 2024, Gmail and Yahoo reject bulk mail that skips this, and Microsoft returns a 550 5.7.x bounce for the same reason.

Reputation is how mailbox providers rate your sending IP and domain based on history: complaint rates, spamtrap hits, bounce rates, and whether your IP has landed on a blocklist. A damaged reputation shows up as 4xx deferrals and spam placement even when your DNS is perfect.

Engagement decides the last mile — which tab you land in and whether you stay out of spam over time. The guides below work through the specific errors and placements you are most likely to hit, each with the exact fix.

Start with your problem

The most common deliverability failures, each with a step-by-step fix.

Why is Microsoft 365 rejecting my email with a 550 5.7.x error?

Microsoft 365 returns 550 5.7.x when your mail fails a policy or authentication check. Fix SPF, DKIM, and DMARC alignment to clear the access-denied bounce.

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What does SMTP error 421 mean and how do I fix it?

SMTP error 421 is a temporary deferral, not a bounce. Fix it by slowing your send rate and authenticating mail with SPF, DKIM, and DMARC before retrying.

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Why do my emails land in Gmail's Promotions tab?

Gmail sorts marketing-style mail into Promotions using content, sender and engagement signals — not spam. Here is how the tabs work and how to reach Primary.

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Why is Gmail rejecting my emails with a 550 error?

Gmail 550 5.7.26 rejections mean your mail fails authentication. See why Google now rejects instead of defers, and the exact fixes to get delivered.

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Why is Yahoo blocking my emails as unauthenticated?

Yahoo blocks unauthenticated mail because SPF, DKIM, or DMARC failed. Fix the 550 5.7.9 'sender is unauthenticated' bounce by authenticating and aligning.

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Why is my email queued and how do I fix it?

Email stuck on 'Queued'? It means your message is waiting to send, not failed. See what causes it in Gmail and on mail servers — and how to fix each one.

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Diagnose your domain in one pass

Run a free Email Security Score to see every authentication and deliverability gap at once, check a sending IP against the major blocklists, or let Palisade monitor DMARC and fix the records for you across every domain you manage.

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Email deliverability FAQ

Email deliverability is whether your messages actually reach the inbox — not just whether they are accepted by the receiving server. It depends on three things working together: authentication (SPF, DKIM, DMARC that proves who sent the mail), reputation (how mailbox providers rate your sending IP and domain), and engagement (whether recipients open, reply, and want your mail). A failure in any one shows up as a bounce, a deferral, or a trip to the spam folder.

A bounce with a 5xx code is a permanent rejection — usually failed authentication, a bad recipient address, or a blocklisted sending IP. A 4xx code is a temporary deferral, often rate-limiting or a reputation throttle that clears on retry. Read the exact code in the bounce message: it tells you whether to fix DNS, clean your list, or slow your sending.

Spam placement is a reputation and authentication verdict, not a content accident. The most common causes are missing or misaligned SPF/DKIM/DMARC, a sending IP or domain with a poor history, high complaint or bounce rates, and sending to people who never engaged. Fix authentication first, then work on list hygiene and engagement.

Start by confirming your authentication is correct with an SPF, DKIM, and DMARC check, then run your domain through a full Email Security Score to see every gap in one place. If mail is being rejected, check whether your sending IP is on a blocklist. Then monitor your DMARC reports so you catch new problems before they hit delivery.