<?xml version="1.0" encoding="utf-8"?>
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml" layout="vertical">
	<!--
	A. HTTPService is used to send the Message
	 -->
	<mx:HTTPService id="sm" url="http://localhost:8081/sendmail" 
		useProxy="false" method="POST" resultFormat="e4x">
		<mx:request xmlns="">
			<msgFrom>
				{msgFrom.text}
			</msgFrom>
			<msgTo>
				{msgTo.text}
			</msgTo>
			<msgSubject>
				{msgSubject.text}
			</msgSubject>
			<msgBody>
				{msgBody.text}
			</msgBody>
		</mx:request>
	</mx:HTTPService>	
	<mx:Panel layout="horizontal" horizontalAlign="center" verticalAlign="middle">
		<mx:VBox>
			<mx:Label text="Flex Client for Google App Engine" fontSize="15" fontWeight="bold"/>
			<!--
			B. Form
			 -->
			<mx:Form defaultButton="{sendMessage}" width="419">
				<mx:Label text="Compose a Mail Message"/>
					<mx:FormItem label="From">
						<mx:TextInput id="msgFrom" width="300" text=""/>
					</mx:FormItem>
					<mx:FormItem label="To">
						<mx:TextInput id="msgTo" width="300" text=""/>
					</mx:FormItem>
					<mx:FormItem label="Subject">
						<mx:TextInput id="msgSubject"  width="300" text=""/>
					</mx:FormItem>
					<mx:FormItem label="Body">
						<mx:TextArea id="msgBody" width="300" height="100" text=""/>
					</mx:FormItem>
					<mx:FormItem>
						<mx:Button id="sendMessage" label="Send" click="{sm.send()}"/>
					</mx:FormItem>
			</mx:Form>
			<!--
			C. Previously Sent Messages Data Grid
			 -->
			<mx:Label text="Previously Sent Messages"/>
			<!--
			D. DataProvider is expecting to receive XML. It is BOUND to  
			 -->		
			<mx:DataGrid id="dg" dataProvider="{sm.lastResult.message}">
				<mx:columns>
					<mx:DataGridColumn headerText="From" dataField="from"/>
					<mx:DataGridColumn headerText="To" dataField="to"/>
					<mx:DataGridColumn headerText="Date" dataField="date"/>
					<mx:DataGridColumn headerText="Subject" dataField="subject"/>
					<mx:DataGridColumn headerText="Body" dataField="body"/>
				</mx:columns>
			</mx:DataGrid>
		</mx:VBox>
	</mx:Panel>	
</mx:Application>

http://www.colettas.org/?p=240

Quick example on how to do a POST request using Flex and the HTTPService class

Flex: Simple Send Mail Application

MXML