I recently changed jobs and need to move nearly 25 GB of archived emails from Thunderbird into my Microsoft 365 account. Everything is stored in MBOX files, including client conversations, invoices, and project discussions that I still need for reference. I initially assumed Microsoft would have a simple import option, but after some research I realized Exchange Online doesn’t recognize MBOX files directly. I tried using Thunderbird with an IMAP connection as a bridge, but dragging thousands of emails folder by folder is painfully slow. A few attachments didn’t sync correctly during testing, and I’m concerned that large folders might fail midway through the migration. Since these emails are important, I don’t want to risk losing folder hierarchy, timestamps, or attachments.

While searching through migration discussions, I found several users recommending methods to import MBOX to Office 365 using dedicated migration software instead of relying on manual workarounds. Many mentioned that manual migration is acceptable for a handful of emails, but becomes unreliable when dealing with years of archived data. What I really need is something that supports batch processing, preserves metadata, and lets me verify the mailbox before starting the migration. A preview option would also help because I want to confirm everything is readable before uploading it into my Microsoft 365 mailbox.

After reading product comparisons, I came across the DRS Softech MBOX Converter Tool, which appears to support direct migration to Office 365 along with features like Preview, Date Filter, Remove Duplicate Emails, and support for multiple MBOX files at once. From what I’ve read, many users choose it when they need to migrate MBOX to Office 365 without worrying about missing emails or broken folder structures. Before I proceed, I’d love to hear from anyone who’s migrated a large Thunderbird or Apple Mail archive. Did you use the manual Thunderbird method, or did dedicated migration software save you significant time?