If you knew the name of the bhikkhu who was reading it, I would write "Greetings venerable x".
If you don't know which bhikkhu will read it, I would write, "Greetings bhante"
I would keep the correspondence factual and to the point.
"Do not force others, including children, by any means whatsoever, to adopt your views, whether by authority, threat, money, propaganda, or even education." - Ven. Thich Nhat Hanh
"Whether I were to preach in brief, Sāriputta, or whether I were to preach in detail, Sāriputta, or whether I were to preach both in brief or in detail, Sāriputta, rare are those who understand." (A I 333, Sāriputtasutta)