Adding Comcast Email to Mac Mail

I've seen an older, very long thread about this issue, and apparently it's still an issue. I've been using Mac Mail for Comcast emails for years without any problems. This worked fine on an older iMac. Now I'm trying to set up my Comcast email on a newer iMac, and I can't. It's not recognizing my email address or password. I've always had the box to allow access to third party programs checked. Xfinity, what is the point of this option if it doesn't freaking work?

I am not going to risk deleting and then adding back my Comcast email, because some posters stated that they couldn't add it back after deleting it. Xfinity should allow customers to make decisions about how to handle their own email and software. Please tell me how to fix this.