Best Way to Transfer Thunderbird MBOX Archives to Microsoft 365?

I have several old Thunderbird mailbox archives stored in MBOX format and recently needed access to them through Microsoft 365. My goal was to centralize all email communication in one place, but I discovered that Microsoft doesn't provide a direct option to open or upload MBOX files.

Initially, I tried a manual solution using Thunderbird with an IMAP connection. While it worked for a few folders, the process became extremely slow when handling larger mailboxes. Some folders took hours to sync, and I constantly worried about missing emails. That’s when I started looking for a better way to import MBOX to Office 365.

During my search, I found the DRS Softech MBOX File Converter, which seemed designed specifically for this type of migration. The software allows users to add single or multiple MBOX files, preview mailbox content, and directly transfer data into Office 365 accounts. What I liked most was that it retained the original folder structure and metadata throughout the process.

Many organizations still maintain years of archived email data in MBOX format. If you're planning to migrate MBOX to Office 365, it's worth considering how much time and effort manual methods can require. A dedicated solution can often save hours of work and reduce the risk of migration errors.

Has anyone here migrated large MBOX archives recently? How did you ensure all emails and attachments were transferred successfully?