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Warning: Cannot modify header information - headers already sent by (output started at C:\Inetpub\wwwroot\store\catalog\includes\classes\email.php:500) in C:\Inetpub\wwwroot\store\catalog\includes\functions\general.php on line 29
How do I rectify this. The SMTP server is working fine.
The 550 error is "Relay Access Denied" and is your mail server telling you that however you have the program configured does not have permission to send out mail on your server. That could be a number of things like having the hostname set wrong in OsCommerce or some other thing that makes your machine think you are an intruder.
I would look at your mail server configuration for send permissions. I have a helluva time figuring all that windows server stuff out so I can't help you there.