I would wrap the letter in <blockquote>
<blockquote>
<p class="noindent">Dear whoever</p>
<p>Some text here</p>
</blockquote>
You can then either define the blockquote in css, or just allow the ereader to style it with its default.
My blockquote css is...
blockquote {
margin-left: 5%;
margin-right: 5%;
}
https://developer.mozilla.org/en-US/...ent/blockquote