How can I get att.net email to delete from att server when deleting on iphone?

Hello,

I currently have my att.net email set up as a yahoo account on my iphone 6. When I delete emails on my iphone they do not delete on the att server causing a baglog of emails when I log into att.net webmail. Is there a setting somewhere to allow my emails to delete from the att server when I delete them from my iphone? Thanks!