How to find an email that was rejected because of a bad/broken DKIM record?

Introduction

If you suspect you have not received an email, because the sender has a bad DKIM record, there is a way to determine it.

You would need to analyze the /var/log/exim_rejectlog file, find the email address in question, and see if you see the DKIM error.

Procedure

1. Log in to the server using SSH or WHM: Terminal.

2. Use a pager on the command-line like 'less' or 'more' to search the following file:

/var/log/exim_rejectlog

3. Search for the full email address you think is being rejected.

Example: cptest@domain.tld

4. If present,  you will see lines like this, meaning the email was coming to the server.

Envelope-from: <cptest@domain.tld>
Envelope-to: <somewhere@your-server.com>

5. When you find the email address, observe the lines right around it. Above the <From> address, you will see a message similar to these:

DKIM: encountered the foll$e following problem validating grupoinditex.onmicrosoft.com: bodyhash_mismatch

rejected DKIM : DKIM: encountered the following problem validating alphanumeric.com: pubkey_unavailable

This shows that a particular email was rejected because of a bad DKIM record. The public key is either wrong or not understood.