This is not so much of a pro tip as a tiny tweak that no one seems to follow.
All popular email clients block images from loading until the user allows it explicitly. The emails, in my opinion, shouldn't look broken till this is done. I work for an e-commerce company and for us, the intent to purchase increases exponentially with images shown. So, there is even greater emphasis to get the user to load images.
The image boxes that most email clients paint are based on the width and height attributes you provide. The text that is shown is the alt text. So, just adding another attribute to your inline style will make emails look much better even before the images are loaded.
<img alt="Flipkart.com" width="600" height="40" src="<image-url>" style="background:#0c528b; color:#ffffff; font-size:14px;"/>