Over the last decade I've used several email clients, including Thunderbird and Apple Mail, so I now have multiple MBOX files containing thousands of important emails. Since my company has moved to Microsoft 365, I'd like to keep everything in one place instead of searching through old archives whenever I need a client conversation or project record. I initially assumed Microsoft would provide a direct import option, but I quickly discovered that Exchange Online doesn't recognize MBOX files. I tested the Thunderbird IMAP workaround, but the process became extremely slow once I started moving larger folders. A few emails didn't sync correctly during testing, and I'm worried that manually dragging thousands of messages could easily result in missing attachments or incomplete folders.

After spending time reading migration discussions, I found that many administrators recommend using a dedicated solution whenever they need to import MBOX to Office 365 instead of depending entirely on manual methods. That makes sense because I don't just need emails—I also need the original folder hierarchy, timestamps, sender information, attachments, and metadata to remain exactly as they are. Since some of these emails are related to legal documentation and customer contracts, accuracy is much more important than simply finishing the migration quickly. Having a preview before migration and the ability to process multiple MBOX files together would also make the entire project much easier.

One solution that repeatedly appeared in recommendations was the DRS Softech MBOX Converter Tool. From what I've read, it supports batch migration, previewing mailbox data before conversion, Date Filter for selective migration, and Remove Duplicate Emails to avoid unnecessary copies. It also claims to preserve the original mailbox structure throughout the migration, which is exactly what I'm looking for. Many reviewers mentioned using it to migrate MBOX to Office 365 without depending on Thunderbird or other intermediate steps. Before I make a decision, I'd love to hear from anyone who's handled a large MBOX migration recently. Did you stick with the manual method, or did dedicated migration software make the process significantly smoother and more reliable?