Be sure to assign one IP to one login. This can be easily done using a hashmap:
HashMap<String, String> loginsandproxies = new HashMap<>();
First string is the login, the second string is the login it's attached too. Once you go through the login, just check if the HashMap contains the login, and if it does, check using that proxy. It'll make the program seem more legit on the Mojang servers, and possibly prevent the locking of accounts.