Server Error - mail could not be sent

[OhioTech]

I have Windows XP and Internet Explorer.
Trying to get email sent thru Apache.

This works:
1. Html file "a" on client's web site. I click on link (href="mailto:emailAddress"). This starts my default email program
Eudora, and starts a new message.
So I have confidence Eudora can be activated.

2. Html file "b" with form that starts php file "c" on client's web
site. This sends mail OK. Don't know what mail server is utilized.
So I have confidence in the html and php code.

But when I run same files "b" and "c" on local Apache, I get this error
message: "Server error - the form was processed successfully, but mail
could not be sent."

This is after I changed these two items in php.ini and restarted Apache:
SMTP = smtp.sbcglobal.yahoo.com (value from Eudora config)
sendmail_from = my email address at sbcglobal

Questions:
Are other php.ini changes needed?
Any other reason mail could not be sent? If file "a" mailto is able to
activate Eudora, why can't Apache?
If Eudora cannot work with Apache, any suggestions for other easy mail
client? I just want to test mail programs on local computer.

Thanks