I have a module in my system that will trigger an email and send it to user. The email will be sent to user when the product in my system is going to expired soon. I test the whole module in localhost and there is no problem with it. Then, I finally moved the module in my server but it gives this error.

sendmail: Error during delivery: Service not available, closing transmission channel. 

It will also create a notepad in my desktop that contains information like this :

command line      : C:\wamp\sendmail\sendmail.exe -t -i
executable        : sendmail.exe
exec. date/time   : 2011-06-18 01:10
compiled with     : Delphi 2006/07
madExcept version : 3.0l
callstack crc     : $fecf9b34, $5562b2fa, $5562b2fa
exception number  : 1
exception class   : EIdSMTPReplyError
exception message : Service not available, closing transmission channel.

main thread ($15b0):
0045918a +003e sendmail.exe IdReplySMTP     501   +1 TIdReplySMTP.RaiseReplyError
0043ff28 +0008 sendmail.exe IdTCPConnection 576   +0    
TIdTCPConnection.RaiseExceptionForLastCmdResult
004402f4 +003c sendmail.exe IdTCPConnection 751  +10 TIdTCPConnection.CheckResponse
0043feba +002a sendmail.exe IdTCPConnection 565   +2 TIdTCPConnection.GetResponse
004403fd +002d sendmail.exe IdTCPConnection 788   +4 TIdTCPConnection.GetResponse
0045ab97 +0033 sendmail.exe IdSMTP          375   +4 TIdSMTP.Connect
004b5f14 +1060 sendmail.exe sendmail        808 +326 initialization
77013675 +0010 kernel32.dll                          BaseThreadInitThunk

thread $cf8:
77a400e6 +0e ntdll.dll     NtWaitForMultipleObjects
77013675 +10 kernel32.dll  BaseThreadInitThunk

thread $1088:
77a41ecf +0b ntdll.dll     NtWaitForWorkViaWorkerFactory
77013675 +10 kernel32.dll  BaseThreadInitThunk

May I know what is the problem of this error ? Is it something like firewall in the server that block my sendmail.exe or anything else ? FYI, I'm using Wamp and Sendmail to send the email. This is my first time seeing error like this. I need an explanation on this. Thank you.