As with all migrations, it's best to analyze and prepare your source data before running the operation. See our recommendations for ShareGate Migrate's Copy mailboxes below.

When planning a mailbox migration, removing any excess data that doesn't need to be copied is essential.

One of the easiest ways to reduce a mailbox's size is to scale down its saved history. In most cases, you can safely reduce a mailbox's saved history to six months. Be sure to confirm your organization's data policies before scaling down your colleagues' mailboxes.

Tip: Some users may use their mailboxes to save important files or information. Give your colleagues ample time to save their data before trimming down their mailboxes.

If you're using copy mailboxes for the first time, copy one average-sized mailbox first to see roughly how many items are copied. Use that number to determine how many mailboxes you can copy at once.

Given we do not recommend migrating more than 800,000 items per copy, if your average mailbox yields 100,000 items, you can copy seven to eight mailboxes at a time.