SendGrid certificate verification failed for smtp.sendgrid.net

TL;DR

Install the CA bundle on the sending server. Tell your mail server where the bundle is (for Postfix, the smtptlsCAfile setting) and restart it.

Steps

1. Symptom: certificate verification failed for smtp.sendgrid.net, untrusted issuer.
2. The connection is still encrypted. The failure is only that your server cannot confirm the certificate chain.
3. Install the CA bundle on the sending server. The doc points at the GoDaddy CA bundle (SendGrid's cert chain) as the missing piece.
4. Tell your mail server where the bundle is (for Postfix, the smtptlsCAfile setting) and restart it.
5. If the server talks to many providers, add the cert to the existing system CA bundle instead of replacing it.
6. Do not fix this by disabling verification. That trades a config task for a real MITM hole.
7. After the fix, a test send over 587 with TLS should complete the handshake without warnings.

When to use

You are seeing this: Symptom: certificate verification failed for smtp.sendgrid.net, untrusted issuer. Use this skill when you run into "SendGrid certificate verification failed for smtp.sendgrid.net".

When not to use

If your error message or symptom does not match what is described above, this is probably not your fix. Search for your exact error text instead of forcing this one to fit.

Versions

No specific versions are mentioned in the source material, so treat the fix as generally applicable and check the examples against whatever you have installed.

Why this happens

The original report does not dig into a root cause. It documents the symptom and the fix that resolved it.

The official SMTP errors doc: 'certificate verification failed for smtp.sendgrid.net ... untrusted issuer' means the connection is still encrypted, but the server lacks the CA certificates to confirm SendGrid's certificate. The fix is installing the CA bundle and pointing the mail server at it. Sources: http://sendgrid.com/docs/Glossary/emailerrormessages.html