Right, finally wrote one. Probably a bit easy, but I felt it was a pretty rhyme:
I send my letters every year,
On parchments yellowed with their age,
I throw them up into the wind,
Unread, they scatter, page by page,
But when you write a letter, then
I write no more and I am gone;
For from my body, limb and heart,
Is made that which you write upon.