I'm making a site for a guy, and he needs a way to email all his customers with a single form to save time and be more efficient. I got the compose page done. But in the past, from my experience, when I sent myself emails generated from sendmail, some email services didn't accept them. They were blocked, didn't even make it to bulk or junk mail folders. This was more than likely because I used a bogus email from address. He has a host account registered, with email addresses to use for the site. But how would I integrate those into a PHP script? Wouldn't I still have to hard code an email address into the sendmail form? How would it know it's real, and not bogus?

I am going to put this into a loop, so that for each customer record that is queried in sequence, an email is sent to their email address. Pretty straightforward.

Thanks in advance.

What function are you using to send email.
You will have to hardcode the sender email address into that function since like you said, you need a valid sender email.
Some Mail Transfer Agents (MTAs) will try to validate sender email before accepting email or callback later for verification. Basically, if you have a valid sender email, then MTA responsible for that address will take care of any validation needed by the remote MTA for which the email is being delivered.