How to say "dear" in the beginning of an email/letter
Hola! I have lessons of Spanish with a teacher from Perú. In these classes [and also in Spanish classes with other teachers] we have always been using the "tú" forms cause the teachers had told us it was the habit on Spanish universities too. And now I'm going to email to this particular teacher and I'm not sure how to start it so that it's appropriate not too formal or too informal. We call her Margarita and use the tú forms... so shall I write "querida Margarita" or is that too personal? or "estimada Margarita"? or something else?
Querida Margarita, Apreciada Margarita or Estimada Margarita. You can use either one.
In Colombia we only use the "Tu" form when we know the person, otherwise we always use "Usted".
I am sure it is, estimado, estimada, estimados or estimadas, depending on the gender and number of people.
Some of my close friends from Peru start letters with "Recordado Arnold"...literally "recordado" is "remembered", and the idea is that they are addressing someone fondly remembered. These are folks above 40, and they have told me that, at least in their circles, "Recordado" is going out of style as the younger folks don´t always carry on the older customs anymore. I have younger friends too, and they use "Querido". I have also seen, as you typed, "Estimado". I think in your case, "Querida" would be just fine, and maybe "Recordada" would be even nicer.
In Mexico letters often begin with Estimado (Estimada) or Querido (Querida)
I do believe "Recordado" is a South American usage, quite an appealing one
Collection of Authentic Handwritten Letters
by Mexican Village Women
to Loved Ones in the United States
For a teacher, I personally would use "negrero(a)", which is:
slave driver (familiar sentido figurado)