What is the safest professional tool to convert Maildir to Office 365?

[arjunmehta]

Desktop utilities such as MacSonik Maildir Converter is used, but they are more suitable for small-to-medium migrations rather than enterprise-scale governance.

For safety, choose tools with Microsoft 365 OAuth support, incremental sync, and audit logging to ensure compliant migration.

Always evaluate vendor reputation, support SLAs, and trial migration results before production use, and avoid low-quality freeware tools that may risk data integrity during Office 365 mailbox transfer. This ensures secure, auditable, and predictable Maildir to Microsoft 365 migration outcomes for enterprise environments now

[davidmithen]

For Maildir to Office 365, first check one thing: is the Maildir mailbox still active on an IMAP server? If yes, then the safest way is not to handle the raw Maildir folder manually. Move the mailbox through IMAP.
You can try Softaken IMAP to IMAP Converter for this type of work. It can move emails from one IMAP account to another IMAP account, so Maildir-based server mail can be moved to Office 365 if both accounts support IMAP login.

The simple flow is:
Maildir server mailbox → Office 365 IMAP account
Add the source IMAP details, add the Office 365 account details, select the folders, and start the transfer. Before running the full migration, test one small folder first. Also keep a backup of the original mailbox data.
One important point: this is for IMAP mailbox transfer, not direct Maildir file to PST conversion.