Welcome to Android Central! Are you using the stock Email app? If so, are you using Yahoo's POP3 or IMAP server? If possible, change to the IMAP server: https://help.yahoo.com/kb/mobile-mail/imap-server-settings-sln4075.html. POP3 is an older protocol, and what you're experiencing may have to do with that.
Have you also considered installing the Yahoo Mail app? It's pretty good.