Hi, I used to do a similar thing, starting over ten years ago, with comcast as my ISP at home. I had a script that would notes to about 200 or so people from a place I used to work at. About 4-5 years ago it stopped working. After trying darn near everything, I did find out that comcast blocked outgoing mail like this on purpose. I assume they still do. It was supposedly a spam issue with them.

Really. But I'm not spamming, so what do I do? Networking and SMTP is outside my area of competence. My mail client works - apple mail, thunderbird, eudora all worked to send mail, so I'm thinking wtf? I changed over to using an applescript thing that sends my notes out through apple mail to each member in a group I set up in Contacts. It's ugly and it's awful, because working in apple script is like learning martian. I still don't get it. But once it worked I stopped caring how it looked, since I wasn't at work.

All this might not make sense on first reading, but it's true. I think you end up bypassing the comcast smtp facility and instead use comcast merely to connect to, say, a google or apple account (I do either, both work).

So long story short - maybe you write a mail client and get around this, or use an existing mail client. But if you figure this out I will be interested to see how it worked.

Good luck to you