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.
3. Search for the full email address you think is being rejected.
4. If present, you will see lines like this, meaning the email was coming to the server.
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.