ActionMailer Question

Is it possible to send email using the SMTP server of my subscriber’s website host? Customers who register at my site should be able send email by using my web app. I would like to use their SMTP server to send any emails that my customers want to send to their clients. I don’t know what the relay problem is, that’s why I want to confirm. TIA.

Thank you for the clarification. That was my original assumption. Now
I can test it to make sure that I don't run into any other problems.