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.