This sample code will display your email in a more readable format in the Gmail app for Android and iOS. This layout will not trigger the grey optimization message in the iOS Gmail app. Also added fix for center alignment in Outlook desktop client.

gmail-optimization.html

<!--[if (gte mso 9)|(IE)]>
<div style="margin: 0px;outline: none;padding: 0px;width: 100%;background-color: #EFEFEF;background: #EFEFEF;font-family:arial,sans;font-size:15px;color:#555555;text-align: center;">
	<table border="0" cellpadding="0" cellspacing="0" width="100%">
		<tbody>
			<tr>
				<td align="center" valign="top" width="100%" style="font-family:arial,sans;font-size:15px;color:#555555;">
					<table border="0" cellpadding="0" cellspacing="0" width="650" bgcolor="#EFEFEF" style="min-width: 650px;font-family:arial,sans;font-size:15px;color:#555555;">
						<tbody>
							<tr>
								<td valign="top" align="left" style="background-color: #ffffff;font-family:arial,sans;font-size:15px;color:#555555;text-align:left;line-height:150px;line-height:1.5;">
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								<div style="max-width:100%;text-align:center;background-color:#FFFFFF;font-family:arial,sans;font-size:15px;color:#555555;text-align:left;line-height:150px;line-height:1.5;">
									<div style="margin:auto;max-width:650px;padding:10px;background-color:#FFFFFF;font-family:arial,sans;font-size:15px;color:#555555;text-align:left;line-height:150px;line-height:1.5;">
										<div style="font-family:arial,sans;font-family:arial,sans;font-size:21px;color:#333333;text-align:left;line-height:150px;line-height:1.5;">This is an email test for iOS GMail app</div>
										<div style="height:20px;line-height:20px;">&nbsp;</div>
										<img src="http://d226aj4ao1t61q.cloudfront.net/sa6lflpet_harmony-650x300.jpg" width="100%" style="display:block; max-width:100%;">
										<div style="height:20px;line-height:20px;">&nbsp;</div>
										<div style="font-family:arial,sans;font-family:arial,sans;font-size:15px;color:#555555;text-align:left;line-height:150px;line-height:1.5;">Topping bonbon jelly-o bear claw. Chocolate cake cake cupcake ice cream sweet roll tootsie roll cake. Cookie sugar plum gummi bears chocolate tart biscuit. Oat cake pastry marzipan cheesecake carrot cake gummies lemon drops. Sweet gingerbread macaroon bear claw wafer gummi bears chocolate cake cheesecake sesame snaps. Toffee pudding donut. Macaroon soufflé jelly beans cheesecake chupa chups bear claw candy dessert. Icing sesame snaps cake marzipan marshmallow chocolate bear claw fruitcake marzipan. Jelly beans pastry ice cream. Tiramisu soufflé candy. Jujubes gummi bears bonbon cupcake. Marzipan macaroon tart brownie gummies sweet bear claw apple pie chocolate cake. Lollipop gummi bears halvah. Apple pie brownie sweet roll donut. Tart jelly lemon drops cupcake sugar plum cake pastry.</div>
									</div>
								</div>
								
								<!--[if (gte mso 9)|(IE)]>
								</td>
							</tr>
						</tbody>
					</table>
				</td>
			</tr>
		</tbody>
	</table>
</div>
<![endif]-->