Have you ever tried moving several gigabytes of email data to Microsoft 365 or Exchange Online? Then you’ve probably already run into Microsoft’s bandwidth limits. To protect its infrastructure, Microsoft applies so-called EWS throttling once a certain amount of data is uploaded or downloaded to or from your mailbox.
While these limits typically don’t affect your everyday email use, they can quickly become frustrating when performing large mailbox migrations. But don’t worry: In this article, you’ll learn all tactics and tried-and-tested approaches to even migrate very large mailboxes smoothly.
Summary:
To migrate large amounts of data to or from Microsoft with MailJerry, start by following our three tips: pick the right time slot, maximise your EWS throttling limits, and minimise other mailbox activities. Next, select the right migration strategy – configure the recommended advanced settings, set up auto sync or backup mode, and for very large mailboxes, consider using date-filtered batches.
➡️ If you run into issues during or after the migration, simply check the log files and refer to our troubleshooting tips to resolve them.
MS 365 Migration:
What is a Large / Very Large Mailbox?
When performing a MS 365 migration, we consider a mailbox to be “large” as soon as the throttling mechanism of the Microsoft Cloud infrastructure takes effect. In general, this happens as soon as more than ~10 GB of email data has been migrated to or from MS 365.
Have a look at the following table to find the category and required or recommended actions that suit your MS 365 mailbox:
| Mailbox Size | Mailbox Data | Migration within MS Limits | Required / Recommended Actions |
| Small | up to 2 GB | ✅ yes | – No further actions required – |
| Medium | 2 - 10 GB | ⚠️ slightly above | Consider increasing throttling policy (view tutorial). |
| Large | 10 - 50 GB | ❌ no | Highly recommended: Increase throttling policy (view tutorial) |
| Very Large | 50 GB+ | ❌ no |
➡️ Exceeds Microsoft’s maximum mailbox size (50 or 100 GB, depending on your plan) Required: Enable MS Archive feature (automatically expands up to 1,5 TB) Required: Increase throttling policy (view tutorial) Highly recommended: Migrate the mailbox data in date-filtered batches |
Before you Migrate:
3 Tips for Large MS 365 Mailbox Migrations
A few simple steps can make your mailbox migration much easier – here are three tips to get you started:
Tip 01:
Plan Your Migration
If possible, schedule the MS 365 migration for weekends or periods with minimal email traffic. Every traffic that is put on the MS 365 mailbox during the migration increases bandwidth consumption and makes you hit throttling limits faster.
💡 Pro tip (when migrating data to a new Microsoft 365 mailbox): Whenever possible, migrate existing email data before the new mailbox is actively used. Fewer mailbox usage means fewer active connections and traffic, which makes your email migration run faster.
Tip 02:
Increase Office 365 Email Limits & Disable Throttling
Follow this tutorial to disable Microsoft’s throttling limits. By disabling MS 365 limits, your migration will be throttled much later. Even when disabling limits completely, Microsoft will limit your mailbox traffic to 150 MB per every 5 minutes. Nevertheless, disabling the limit helps you to move more data more quickly to or from Microsoft’s servers.
Tip 03:
Minimise Other Mailbox Activities
During the migration, avoid running additional jobs like backups or syncing IMAP clients. If such tasks are scheduled, pause them temporarily. This ensures that you can use all of your available bandwidth for the migration.
⚠️ When migrating to or from Microsoft 365/Exchange Online, don’t run other migration tools or email clients alongside MailJerry. Doing so could trigger Microsoft’s throttling limits and close your IMAP connection.
Setup:Select the Right Migration Strategy
For large and very large data migrations to or from MS 365 / Exchange Online, you need to identify the migration strategy that works best for your use case. Have a look at these three variables to find the perfect setup:
Variable 01:
Configure the Advanced Settings
Before starting your migration, open the advanced settings on the summary page in MailJerry. Here, you can enable options to reduce bandwidth consumption between servers and enable MailJerry’s migration cache. To identify the configuration that works best for your mailbox, have a look at this table:
| Scenario | Enable “Reduce Bandwidth” | Enable Mailbox Cache |
| Your new mailbox is empty. | ✅ recommended | ✅ yes |
| Your new mailbox already contains emails. | ❌ not possible | ✅ yes |
| The migration target is a MS 365 mailbox with enabled archive. | optional | ✅ yes |
| Deleted emails should not be synced after the migration. | optional | ✅ yes |
💡 Pro tip: Depending on the amount of email data and the time available for the migration, consider setting a daily upload and download limit in the advanced settings. This will slow down your migration, but helps to stay within Microsoft’s throttling limits.
Variable 02:
Enable AutoSync or Backup Mode
Even when disabling Microsoft’s throttling policy, it is still possible that Microsoft closes the connection to your mailbox during the migration. Therefore, it’s a good idea to enable the auto sync in the migration settings. In case your migration is disrupted by Microsoft, the auto sync will try to establish a new connection every three hours and migrate the missing data.
➡️ Of course, email data that has already been migrated will not be migrated again.
💡 Tip: You can also enable or disable the auto sync after the initial migration. Simply select your migration in the overview and scroll down to “options”.
Troubleshooting: What to do if Disconnects Remain
For mailboxes that experiences lots of traffic, it’s possible that the auto sync tries to re-connect too quickly with Microsoft, giving the mailbox limits not enough time to recover. So in case your auto sync keeps failing, enable the backup mode instead and set a daily interval. This will start a new sync every day, giving Microsoft’s data limits enough time to recover.
⚠️ Important: Don’t forget to disable the auto sync when using the backup mode!
Variable 03:
Migrate Very Large Mailboxes in Date-Filtered Batches
If your mailbox is very large (50 GB or more), consider splitting up the migration into date-filtered batches.
Example:
- Migration 01: Emails from 2016 up to 2020
- Migration 02: Emails from 2020 up to 2024
- Migration 03: Emails starting from 2024
➡️ Date-filtered migrations work especially well with Microsoft’s archive feature, as your Microsoft mailbox receives enough time to move incoming data from the inbox to the archive.
Tips for date-filtered batch migration:
- After every batch, wait 12 to 24 hours before starting a new batch, so that your MS 365 mailbox has enough time to move data to the archive and reset throttling limits.
- Enable the auto sync / backup mode only for the last batch to prevent unnecessary disconnects.
Learn here how to migrate emails by date:
TroubleshootingAfter the Migration
After your migration completed, it’s a good idea to check your log files for possible problems: Select your migration in the overview and click on “Download Log Files”. When migrating large and very large mailboxes to or from Microsoft, there are two common errors that may occur:
Common Errors
Reported by Microsoft
Trying command when NOT connected!➡️ This error may occur repeatedly. It means that Microsoft closed the connection to your mailbox.
Request is throttled. Suggested Backoff Time: 299961 milliseconds➡️ This error is usually fired only once, followed by many “trying command when NOT connected” errors. It means that your migration exceeded Microsoft’s throttling limit.
How to Resolve These Errors:
In case your log file contains one of the two errors above, you need to perform a new sync by clicking on the button “Sync Now” in the options of your migration. If this does not resolve the issue, disable the auto sync (if enabled) and activate the backup mode with a daily interval. Your migration will automatically resume after 24 hours, giving Microsoft’s systems time to reset the exceeded limits.
What to do if
Your Migration Keeps Failing
You followed the recommendations above, but your migration still shows errors? Try the following solutions:
✅ Solution 01: Give your migration a little more time
If you sync your mailbox frequently (by clicking on “Sync Now”) or if the auto sync is enabled for a very large mailbox, it’s possible that you’re still hitting Microsoft’s daily download limits. Give your migration more time by waiting 24 hours before starting a new sync – and don’t forget to disable the auto sync! Alternatively, enable the backup mode with a daily interval so the migration resumes automatically.
✅ Solution 02: Try again with different settings
If problems persist, consider deleting all data in your new mailbox (if possible) and start a new migration. This time, enable the option “Reduce Bandwidth Consumption” (requires the source mailbox to be empty).
✅ Solution 03: Check the source mailbox
Sometimes the source mailbox contains corrupt emails, which can cause issues when migrating to Microsoft 365 / Exchange Online. Delete corrupt emails before performing a resync. MailJerry’s log file will help you identify these problematic messages.
‼️ Solution 04: You’re a rare case
Your migration is still not working? If so, we’d love to investigate. Contact our support team so we can review your migration and test it with different settings. Please only do this if you’ve tried all suggestions above 🙂.
