Gmail 550 5.7.40: no DMARC record for the sending domain

Short answer: Your domain has no valid DMARC record, which Gmail requires from bulk senders. Fix: publish one TXT record at _dmarc.yourdomain.com such as v=DMARC1; p=none; rua=mailto:you@yourdomain.com.

The bounce looks like this
550-5.7.40 Your message was blocked because the sending domain doesn't have a DMARC record or the DMARC record doesn't specify a DMARC policy.

What it means

Gmail requires bulk senders to publish DMARC. Your domain has no TXT record at _dmarc.yourdomain.com, or the record is missing the p= tag (or is broken, e.g. two DMARC records).

How to fix it

  1. Create the record with the DMARC generator: host _dmarc, value starting with v=DMARC1; p=none.
  2. Add a report address (rua=) so you can see every service that sends as your domain.
  3. Make sure there is only one TXT record at _dmarc, then run the checker below.

Check your domain now

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