From: sender@example.com
To: recipient@example.com
Subject: =?CP1251?B?T2zpcg==?=
Content-Type: multipart/alternative; boundary="boundary-string"

--your-boundary
Content-Type: text/plain; charset="utf-8"
Content-Transfer-Encoding: quoted-printable
Content-Disposition: inline

Plain text email goes here!
This is the fallback if email client does not support HTML

--boundary-string
Content-Type: text/html; charset="utf-8"
Content-Transfer-Encoding: quoted-printable
Content-Disposition: inline

<h1>This is the HTML Section!</h1>
<p>This is what displays in most modern email clients</p>

--boundary-string--