It looks like just about any Yahoo related service is either down or running painfully slowly this morning. Looks like they have major issues.
BBC covering it now ... https://www.bbc.co.uk/news/technology-49596820
I've got my Yahoo accounts linked to my Gmail account and attempts to retrieve Yahoo emails have resulted in this message:
Error while reading mail.
Server returned error "Error in LIST command: [SYS/TEMP] Server error - Please try again later."