Additionally, some servers don't like plaintext connections, so --ssl1 and --ssl2 flags can be handy:
./imapsync --nosyncacls --syncinternaldates --ssl1 --ssl2 --host1 servername1 --user1 username1 --password1 password1 --host2 servername2 --user2 username1 --password2 password2 --authmech1 LOGIN --authmech2 LOGIN |