Fix false-positive 'Checksum mismatch detected' warnings

I noticed that `Checksum mismatch detected` warning is shown in non-malformed encrypted email messages for senders with correct encryption keys. Need to find why these warnings are shown for valid messages and improve our checksum validation flow.

<img width="628" height="138" alt="Image" src="https://github.com/user-attachments/assets/e711e018-51d8-498e-ba37-b54ab8c3b1a0" />