Ayt. Sistanis Office:
1. If you have learnt to pronounce the letters well and it was just a mistake that day - Namaz is valid.
2. If pronounced incorrectly on purpose - Namaz is Void.
3. If there was an opportunity (Class/Personal Teacher) where you could go and learn the proper pronunciation of the letters and did not make the effort to do so and kept reciting Namaz the way you know - Namaz is Void.
Ask An Alim Team.