Can I point out that if you do start to have a significant users whose mail is instantly going to Spam - It might be worth having your email team reach out to Yahoo to see if there is anything they can do.
All email providers have a Postmaster address. In addition some have postmaster tools. These are used for the postmasters belonging to one domain to raise issues with the postmasters belongng to another.
As an example - When I set up my mail server, The Server was being blocked from sending Mails to Outlook.com addreses. I contacted the postmaster team at Microsoft in order to get the block lifted.
Similarly Yahoo have a page that your postmasters can use in order to contact them.