Here some sample math formulas in plain text, suitable for 
any e-mail program:

  The Pythagorean Theorem:  a^2 + b^2 = c^2

  Subscripts:  a_1 + a_2 + ... a_n

  An integral: \int_0^1 x^2 dx = 1/3

Not pretty, but it gets the idea across.