Go to Ayarlar | Bildirimler to see your accounts and their notification status (e.g. Etkin).
If battery optimization is interfering with notifications, you'll see a warning at the bottom of this screen: Pil Optimizasyonunu KAPAT. Tap Şimdi Ayarlara Git to fix it directly from there.
For more detail on excluding BlueMail from battery management, see Battery Optimization.
