So I have been emailing my future employers (for an internship) for about a 3/4 of the year which has resulted in many emails being exchanged. When I first responded to their email, I addressed the recipient as Mr. Employer. Using Mr. In general makes me a little uncomfortable because I don't want to appear brownnosy, but also don't want to offend anybody and also would like to appear polite by using Mr. So I would use mister in an email, and hopefully don't have to respond many more times to continue the gesture.
Is it ok or expected to switch to first name over the course of a lengthy email chain? And should I have even used "Mr" in the first place?
Edit: I am based in the US