Im getting a very similar issue with my server.
When sending to a email account on the server it all works fine. To any external domain it does not.. First noticed the issue as I was not getting monitoring emails to externally hosted email box.
Now php forms and user accounts on site are unable to send validation emails, passwords etc to member accounts.
IP is static and ISP do not filter smtp connections and I am sure of this as I work for them :P
ISPC 2 latest install/version
Centos 5.2 - Perfect Server Installation
Now I am a noob to email server stuff so you will not only have to tell me what I can give you to further assist but also where I can find it.
I originally suspected global php settings to be the issue but not after reading others comments, now suspect postfix or ....
Originally Posted by misiek04
Send to another server. I solve this problem. In /etc/mailname change hostname.mydomain.com to mydomain.com. Now everything works.
...to be the issue.
Any help is very much appreciated.