Correspondece chess on agreed openings I would like to play correspondace chess, but in order to use it as training for my openning repertoire, I want to agree with the oponent which opening to use.

For example, We bith give a list of openings/variations and agree in which position start playing. Is any website with this functionality?
Gameknot allows mini-tournaments to specify a starting position for all games in the tournament. This is a popular way to explore openings; I do it frequently.

Mini-tournament participation does require that you've completed a few games on the site already though.