Emails are just one part of the whole picture. Emails alone are not too indicative of anything. Coupled with Instagram follows, club visits, mail, etc. and a strong picture develops. So if several of those things are all happening then you have a better sense of where you might stand in their recruiting list.
In regards to when to send out mails? Our coach prior to the season took the 2027s aside and asked for their target colleges. The coaches then called the programs directly and began a conversation regarding the gymnast and their interest in the program, sending videos, giving background information, etc. The coach then coordinated if there was mutual interest for mail, club visits etc. If you are in the upper tier of your recruiting class (4 star, maybe even a very high 3 star to 5 star) I am not sure you need to worry too much about sending out emails. There is enough videos online, etc that email has almost become archaic. And if you are not, then you are probably a tier B or C recruit. I would wait till June 15 when colleges can directly speak to you.