It appears my ISP (Spectrum) requires authentication. They mention to "check the box for authentication" in the setup instructions for outgoing mail but my older Android phone's email app has no such feature. I also have no problem with incoming email, and Spectrum's webmail site also works fine, it's just much easier to use the phone's built-in email app because it looks like MS Outlook. I was told webmail doesn't need authentication. Here's another one, my app works perfectly using home WiFi, which means the issue is AT&T. I called everyone, AT&T, my ISP, and Samsung. Of course, they all say it's one of the other's fault. One time I did get resolution from Spectrum, they said it's a problem going through third-party apps. It only worked for a short time. I think they don't want to tell me they only support their webmail. I'll buy a phone with a screen large enough to easily use their webmail if that's my only choice. I'm not fond of Gmail.