Anyone else migrating a large batch of IMAP mailboxes to Office 365? Doing 100+ accounts one at a time isn't realistic, so I went looking for something built for scale rather than a single-mailbox tool stretched thin.

I landed on the DRS Softech IMAP to Office 365 Migration Tool, mainly because it supports bulk migration via CSV — you add multiple IMAP accounts at once instead of logging in individually.

A few things stood out for large jobs specifically:

Since big batches rarely run perfectly in one go, "Skip Previously Migrated Items" lets you pause and resume without duplicating data. There's also a duplicate removal option (based on To, From, Subject, Bcc, Body) for cleanup. If you'd rather split the job, the mail filter (date, subject, sender) lets you migrate in smaller chunks, and scheduling lets you run tasks once, daily, weekly, or at a custom time — useful for spreading things out.

Log and temp path settings let you track activity during a long migration, and attachments, inline images, formatting, and folder hierarchy all carry over correctly.

One Note: the free demo only covers 50 emails per folder, so for real scale you'll need the licensed version, which includes 24×7 technical support.
If anyone's actually pushed 100+ mailboxes through this or a similar tool, would love to hear how it went.