You’re sending e-mail, but receive the Mailer-Daemon error message that you’re spammer. Some mail-servers don’t trust emails, which have different primary hostname and original domain in headers. For example you have WHM based on abc.com, and cPanel account on def.com.
Please, edit /usr/local/cpanel/etc/exim/perl/mail_permissions under root account. Change the line $primary_hostname ||= Exim:expand_string(‘$primary_hostname’); to $primary_hostname = Exim::expand_string(‘$original_domain’);