[Exim] MX record as a alias vs cname

The following error showed up one day. Is this part of a standard or
just a local admin's setting? Yes, I have looked for the answer myself
first. Thank you for you help.

  SMTP error from remote mailer after RCPT TO:0000000000000000000:
     host 00000000000000 [IP]: 550 invalid MAIL-FROM:
     Error during DNS A lookup for 00000000000000000:
     DNS alias found where canonical name wanted [Irritated]