How do I prevent this address change from occurring?

6 Dec 2018 07:33:03 Processing mail from someuser@YAHOO.COM for TEST

DMARC> v=DMARC1; p=reject; pct=100;;

DMARC> YAHOO.COM has p=reject

6 Dec 2018 07:33:03 "From:" address rewritten due to DMARC rule

> Old address: someuser@YAHOO.COM

> New address: 0000003942b2b314-dmarc-request@LISTSERV.EXAMPLE.COM

In reality, you probably don't want to.

LISTSERV rewrites the ‘From:’ address of (in this case) addresses because has a DMARC ‘p=reject’ policy, meaning that any messages received from a non-Yahoo mail server that have a Yahoo address in the ‘From:’ header will be rejected by many sites (including virtually all of the big mail providers). Additional details regarding this rewriting can be found here:

Failing to rewrite the ‘From:’ address would cause the message to be rejected by mail servers with DMARC "p=reject", and increases the probability that your LISTSERV installation and mail servers will be seen as a source of spam, damaging their reputation, and increasing the change that other mail from your server will also be flagged as spam.