I use Wordpress + awpcp (a plugin for classifieds/ads).
The users/owners who use hotmail don’t receive emails, why ?
Maybe for this? (i don’t understand):
Diagnostic-Code: smtp; 550 SC-001 (COL0-MC4-F10) Unfortunately, messages from 18.104.22.168 weren’t sent. Please contact your Internet service provider since part of their network is on our block list. You can also refer your provider to http://mail.live.com/mail/troubleshooting.aspx#errors.
I use email@example.com.