How to move email folders from old Yahoo account over to ATT account

Unfortunately though I haven't used my old Yahoo address in decades....when the email accounts were just split by AT&T....they sent all my folders and e-mails (all of which were sent to me and stored with my @att.com account, not my @yahoo account) to the yahoo account and those folders and e-mails are not in my AT&T email account.  Is there any way to get them over?  I can add and see both but I don't care about keeping the old Yahoo account and don't want to manually recreate the folders and e-mail them one at a time to get them over.  Help.