Thanks, I wanted to know what else you needed to help me.
I have an account through my webpage (deedon.us). It is a plesk server. All of my emails go out through the same account, obviously with the same settings. Only emails sent to this one domain (craigslist) kick back with the message. My ISP is Frontier but I am not using the email address provided though them.
My server name (incoming and outgoing) is mail.deedon.us. This is a POP account (at least the incoming is).
In the advanced section of "Edit SMTP Server List" I have "Use default ports (25, 465, 587)" marked.
I also have "Use Secure Sockets Layer (SSL)" checked.
The Authentication option is "Password."
What part of the configuration would I change and would it affect the outgoing emails that are not problematic?
Sorry, but I've never heard of deedon.us. My best guess would be to contact their customer support.
Various sites, including Craig's List, use various tricks to combat spam. It could be related to that.
You could try switching from "Use default ports," and manually entering 465 and then trying 587.
You may also be able to configure the SMTP settings to use the Frontier server.