What OS is your server? I'm running IIS 6 on Win2K3 with php 5.1.1
Below is a snippet from my php.ini
[mail function]
; For Win32 only.
SMTP = server.domain.com
smtp_port = 25
; For Win32 only.
; commented out as the wiki settings provide this value.
;sendmail_from = wiki@domain.com
Below is a snippet from my local.php
$conf['mailfrom'] = 'wiki@domain.com';
NOTE: The php.ini sendmail_from is commented out.
I'm not sure if you tried this, but you can create a simple php page to validate all of your php variables. I call mine php_info.php Validate your SMTP variable is getting picked up.
<html>
<head>
<title> PHP Information </title>
</head>
<body>
<?php
phpinfo( );
?>
</body>
</html>
You could also backup your old server's Dokuwiki install and copy your new one to it. If it works, it's a server/php issue. If it doesn't it's a Dokuwiki configuration issue. This might help narrow your troubleshooting down.
Good luck!