\documentclass{letter}
\signature{Tom Ford}
\address{Balliol College\\ Oxford University\\ Oxford\\ OX1 3BJ}
\begin{document}

\begin{letter}{Sticky\\ Compsoc Network\\ Somewhere in Trinity}

\opening{Dear Sticky,}
Are you enjoying your semi-retirement?\\
For no reason here's an equation:

\[
\frac{sin x}{x} = 2
\]

and some more random text.

\closing{Best Wishes,}
\end{letter}

\end{document}