>>Thanks everyone for all the advice here. I finally was able to test my code yesterday using an email address hosted on Dreamhost (one I control, but wouldn't normally think to use for something like this). Once I filled in the right SMTP info, my original code worked and I've been able to move forward in giving my client what he needs.
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>And what was the sticking point, i.e. the cause?

I think just different ISP's rules. I needed to keep trying until I found an SMTP server to test with that didn't require some complicated extra steps in validating, but would let me sent with just the sender's log-in credentials.

And yes, I'm aware this means that my client's customers may run into issues, but we'll cross that bridge when we come to it and with luck, IT people will get involved to set up what's needed to allow them to have the application send email.

Tamar