Good day, I've tried searching for this very issue on the forums, but saw that there were none. Hopefully, this thread might help me and any others in the same situation. One of my users started experiencing an issue where, whenever they try replying to certain email chains, the attached error message pops up, and it does not allow them to reply to the thread. I tried troubleshooting with the user in my free time, and these were our findings: - It starts happening after the thread is over 5 messages long. - They are the only one experiencing the issue out of all my other users. - We tried sending with and without attachments and changed some wording in the email to no avail. - We removed and played around with the people in the "CC" and "To" fields. - Thunderbird is up to date. The only fix so far is to just start a new thread and continue communicating on there. Would appreciate any assistance.