IMAP settings for att.net account on IPhone 5c

I have an IPhone 5c

I use this for my att.net email account..

the current IMAP/SMTP Settings are:

Incoming: imap.mail.yahoo.com

Outgoing: SMTP:mail.yahoo.com

Server Port 465

I am able to receive emails on this account and now I am unable to send any email from this account? I am thouroughly confused..

I have another att.net account and the email sends and recieves fine on that one.. The setting looke the same as well.

I have tried deleting the account and adding it back in twice and nothing..

Anyone have any solutions? Please??

Tracy

[edited for privacy – please do not post personal information]