Exim4 client rejecting TLS certificate

The relevant error is "The certificate issuer is unknown".

That usually means one of several things:

• The certificate is self-signed and therefor not trusted.
• The certificate is signed by a CA, but the CA that issued the certificate is not known to the client
• The certificate is signed with an intermediate certificate of a known CA, and although the CA is known to the client, the server is not including the necessary intermediate certificate, breaking the CA validation chain.

Then looking at the certificate output you see: issuer CN=mail.reformaspaco.es

and you know the cause of the error is that the first reason.

You will need a proper certificate to get rid of that error.