SMTP messeges struck in queue folder

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IIS6

IIS

SMTP

I have hosted my application in IIS.

In my application email alerts option is there.

When application sends emails all emails are struked in queue folder.

My smtp settings in web.config

<system.net>
		<mailSettings>
			<smtp>
				<network host="localhost" port="25" userName="" password=""/>
			</smtp>
		</mailSettings>
	</system.net>

I have tried with telnet testing also but no luck.

Can anybody please help me