Bug: Email threading broken in some cases

[rubenhensen]

Description

There are instances where encrypted emails are not properly kept in an email thread. The exact cause is unknown.

Steps to reproduce

• Need to identify a reproducible scenario where threading breaks
• Investigate what headers or metadata might be lost/modified during encryption that affects threading (e.g., In-Reply-To, References, Message-ID)

Goal

1. Create a reproducible bug report
2. Identify root cause
3. Fix threading so encrypted emails stay in the correct conversation thread