URGENT – Nine months of emails disappeared from an IMAP folder (January–September 2026)

Hello, I urgently need technical assistance with a serious issue involving Mozilla Thunderbird and my professional certified email account (Aruba PEC, Italy). I have been using Thunderbird to manage my professional email correspondence for several years. I organize processed emails into yearly folders (2024, 2025, 2026, etc.), located under my email account in Thunderbird, not under Local Folders. Today, October 13, 2026, I discovered that all emails dated January through September 2026 have disappeared from my "2026" folder. Only emails from October 2026 remain. The missing messages include professional correspondence, certified email delivery receipts, acceptance receipts, and potentially legally significant communications. What I have verified so far: The "2026" folder exists both in Thunderbird and in Aruba Webmail. Both display the same remaining emails from October. The Trash folder is empty. The 2024 and 2025 folders still contain their messages. Thunderbird's retention settings are configured to "Don't delete any messages." The "2026" folder inherits the account's retention settings. I did not knowingly delete or move these messages. Aruba customer support informed me that they cannot restore the missing emails. I checked Windows Previous Versions and OneDrive, but no backup was found. I located the local Thunderbird mailbox file named "2026" in the profile's ImapMail directory. It is approximately 3 MB. Inspection of a copy of this file did not reveal message headers from before October 2026. I could not find another larger copy of this mailbox file in the Thunderbird profile. I occasionally remove unused folders under "Local Folders", but I do not recall deleting any folder containing these messages. The affected "2026" folder is located under the IMAP email account. My questions are: Could Thunderbird have deleted or moved these messages automatically despite the retention settings? Could IMAP synchronization, folder corruption, message filters, or compaction explain this situation? Are there any additional Thunderbird files, caches, databases, or recovery methods worth investigating? Is there any way to identify when or how the messages were deleted? What steps should I take to prevent further data loss? I have preserved a backup copy of my Thunderbird profile and would prefer not to perform potentially destructive operations. This is particularly serious because the missing messages are part of my professional and legally relevant correspondence. Any technical guidance would be greatly appreciated. Thank you very much for your help.