Hey Kris !
I have found a work-around to get Yahoo to work with Mi Mail. Give this a try ... it worked well for me on the AU account.
1. Login to Yahoo mail on Internet webpage
2. Go to your Account > Account Info
3. Select Account Security on Left Hand Side
4. Re-enter your normal Yahoo security info to validate ID
5. Scroll down to "Other Ways to Sign In"
6. Select Generate and Manage App Password
7. Add temp password Name (I just used "Mi Mail" as reference)
8. Password will be generated (don't add spaces when adding password to your Mi Mail setup)
9. Set up Yahoo account in Mi Mail on your device using manual IMAP settings (as below) and use the generated password from Yahoo
10. Everything should function correctly
Cheers!
Lee
Incoming Mail (IMAP) Server
Server - imap.mail.yahoo.com
Port - 993
Requires SSL - Yes
Outgoing Mail (SMTP) Server
Server - smtp.mail.yahoo.com
Port - 465 or 587
Requires SSL - Yes
Requires authentication - Yes