Troubleshooting

Gmail SMTP error 550-5.7.26: how to fix it

If you are sending email to Gmail and getting back "550-5.7.26" or similar SMTP errors, it usually means your email is failing authentication. Here is how to diagnose and fix it.

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What does 550-5.7.26 mean?

This error code means Gmail has rejected your email because it failed one or more authentication checks. Common variants include:

550-5.7.26: Message rejected due to SPF failure
550-5.7.25: Message rejected due to DMARC failure
550-5.7.1: Message rejected due to DKIM failure

All of these point to the same root cause: your domain is not properly authenticated.

Step 1: Check your SPF record

SPF tells Gmail which IPs are allowed to send email on behalf of your domain. If your sending IP is not in your SPF record, Gmail will reject the email.

Run this command to check your SPF record:

dig +short TXT yourdomain.com

Make sure the SPF record includes your sending service. For example, if you use Google Workspace, your SPF record should include include:_spf.google.com.

Step 2: Check your DKIM signature

DKIM signs your email with a private key, proving it was sent by you. If your DKIM signature is missing or invalid, Gmail will reject the email.

Run this command to check your DKIM record:

dig +short TXT google._domainkey.yourdomain.com

Make sure the DKIM record exists and matches the selector your email provider uses.

Step 3: Check your DMARC record

DMARC ties SPF and DKIM together and tells Gmail what to do with unauthenticated email. If your DMARC record is missing or misconfigured, Gmail will reject the email.

Run this command to check your DMARC record:

dig +short TXT _dmarc.yourdomain.com

Make sure the DMARC record exists and has a valid policy (p=none, p=quarantine, or p=reject).

Step 4: Check your IP reputation

If your sending IP is on a blocklist, Gmail will reject your email regardless of your SPF, DKIM, and DMARC configuration.

Run the free InboxProof audit to check your IP reputation against major blocklists.

Step 5: Verify the fix

After making the changes, run the free InboxProof audit to confirm all three protocols are configured correctly. The audit checks:

SPF record exists and is valid
DKIM key is published and strong
DMARC record exists with a reporting address
Alignment between your "From" domain and the SPF/DKIM domains

Fix your Gmail SMTP error

Run the free audit and see if your SPF, DKIM, and DMARC are configured correctly.

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