The Sectigo intermediate root these new certificates use is from November 2018, so if the sending system is older and hasn't updated its trusted roots bundle then it won't trust this certificate.
For "454 4.7.5 Certificate validation failure" Error - please refer to https://support.sectigo.com/articles/Knowledge/Sectigo-Intermediate-Certificates
Ticket reference : https://support.duocircle.com/a/tickets/403951