I have a program to send email written in PHP, it always works, but I just checked it, it cannot send email any more. Domain name:
www.friendcity.org
Is there any change to the mail setting?

No there are mail settings still the same.

Actually I can send email by using php program before but it suddenly down on the server. Is there any update or any change? Can you check it?The same program , I have not changed it.

I got the message if I use mail() in PHP

Warning: mail(): SMTP server response: 503 This mail server requires authentication. Please check your mail client settings

If I use @mail() in PHP, the message will not send out.
My program works before, you should have changed the SMTP setting on the server recently for www.friendcity.org. This is caused by the wrong setting in php.ini. I checked the technical manual for this. Please refer to the post:

http://www.webhostingtalk.com/showthread.php?threadid=225916

Maybe you can fix this problem by change back the setiing in the ini file.

Thanks. have a nice day.

It is fixed now.
It was caused by anti-spamers software, that detected spamer on our server.
We move out spamer from server and enable mail again.