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
"The uprooting of identity is seen by the noble ones as pleasurable; but this contradicts what the whole world sees." (Snp 3.12)
"You've got to focus on what's really worthwhile in life, which means resisting a lot of the currents in our culture" (Thanissaro)