2020 ATT email upgrade not working in mail

I have tried, repeatedly, to change my mail program to send/receive my ATT email account after their recent 'security' upgrade. The way I read the info, it would appear as though I do not need to get their separate key word, because I am running High Sierra 10.13.6. Is this correct? Any suggestions or help would be much appreciated. Right now I am able to send and receive on the currently site, but not through the apple mail program with that account. I am receiving my other account on mail, though it will not let me send with it either. That's another issue, I guess. I was doing a work around before, by sending with one and receiving both. Now I'm just frustrated. Apple communities rock, and will help me, I know!