Why the external email address was changed when sending to lists members such as Yahoo, hotmail?

Some big mail providers such as yahoo or hotmail implemented the DMARC (Domain-based Message Authentication, Reporting & Conformance) spam & phishing protection. In order to deliver message to lists members with these external domains, recipient's display address has been modified in the message:

e.g. Kevin Ko <kevinko1234@yahoo.com> will be modified to Kevin Ko <kevin1234@lists.ust.hk>

Since the Reply-To address remains unchanged, you can simply reply to the list members as usual in spite of the changed displayed email address.