Migrating large mailboxes with several GB of data from Gmail to MS 365 requires proper planning – and for good reason:
⏱️ Both providers apply bandwidth limitations and throttling mechanisms to protect their cloud infrastructure and ensure stable performance for all users. Once these limits are exceeded and throttling is triggered, the IMAP connection to the mailserver is temporarily closed, which usually interrupts the email migration.
🗂️ In addition, Gmail’s label-based system differs fundamentally from Microsoft’s folder-based structure. Labels must be converted during the migration, which can lead to duplicates or unexpected mailbox structures if not handled properly.
➡️ In this article, we’ll first share three tips to prepare for your large-scale mailbox migration. Furthermore, we’ll walk you through two different migration strategies so you can choose the one that best fits your requirements. And don’t forget to have a look at our extra tips and troubleshooting advice to avoid common issues and ensure a successful email migration from Gmail to Microsoft!
➡️ What is a large mailbox?
When migrating from Gmail to Microsoft 365, we consider any mailbox over 10 GB as “large.” You can find more details in our mailbox size overview — but even if your mailbox is only above 2 GB, the steps in this article will help to seamlessly move your data from Gmail to MS 365.
Summary:
To perform a large-scale migration from Gmail to MS 365 or Exchange Online, we recommend to follow our three tipps to prepare for the migration. After that, opt for one out of two migration strategies: Skip duplicates or migrate duplicates. Selecting the right strategy helps to convert Gmail labels into folders according to your requirements. Before starting the migration, you might also consider having a look at our additional setup tips.
➡️ By following the instructions in this tutorial, you’ll be able to easily migrate mailboxes of several GB of data seamlessly from Gmail to Microsoft. And in case you run into problems, our troubleshooting tips help to easily resolve any issues.
Before You Start:
3 Tips For Large-Scale Gmail to MS 365 Migrations
Before you start migrating multiple GB from Gmail to Microsoft 365, make sure to follow these three tips:
Tip 01:
Check Your Storage Limits
Microsoft 365 mailboxes are limited to 50 GB on Business Standard and Premium plans, and 100 GB on Enterprise plans. In comparison, Gmail mailboxes can range anywhere from 30 GB up to 5 TB, depending on your subscription.
➡️ So it’s always good to check if your Gmail data doesn’t exceed the capacity of your Microsoft mailbox.
💡 My Google mailbox exceeds the size of my MS 365 plan. Can I still migrate?
Yes! You can still migrate with a workaround. Here’s what you need:
- A MailJerry Yearly Plan (which allows to migrate your data in date-filtered batches)
- A Microsoft plan with an online archive that is big enough to store all emails from your Google mailbox:
- Business Basic: 50 GB + 50 GB Archive (fixed size) or
- Business Premium: 50 GB + 100 GB Archive (auto-expands up to 1.5 TB) or
- Enterprise: 100 GB + 100 GB Archive (auto-expands up to 1.5 TB)
Tip 02:
Increase Office 365 Limits
As mentioned earlier, Microsoft applies EWS throttling to Office 365 and Exchange Online mailboxes as soon as a certain data limit is exceeded. Since these limits can cause disconnects during the migration, it’s best to disable Microsoft’s throttling limits before starting the migration.
Even after disabling Microsoft’s limits, traffic is still limited to 150 MB every 5 minutes. However, disabling EWS throttling will help you move your data more quickly and will significantly cut down the time required for the email migration. Simply follow this tutorial to learn how to disable throttling.
Tip 03:
Avoid Additional Load on Your Mailbox
To keep your migration running smoothly, don’t use other email tools, backups, or IMAP clients while MailJerry is migrating your emails.
⚠️ Running multiple jobs on your mailbox at the same time can trigger Microsoft’s throttling limits and close your IMAP connection, even if EWS throttling is disabled. Therefore, if possible, pause all other mailbox activities to ensure that your migration is performed without interruptions or delays.
➡️ After following these three preliminary tipps, it’s time to find the right setup to easily move several GB from Gmail to MS 365.
Setup:Choose Your Migration Strategy
When migrating from Gmail to Microsoft 365, it’s important to remember that Gmail uses labels instead of folders. So when migrating your email data, these labels need to be converted.
To provide compatibility with other providers, Gmail “maps” each label into a folder, and since a single email can have multiple labels, it may end up being copied into several folders, creating duplicates. This behaviour is unique to Gmail and requires to decide in advance how to handle these duplicates.
Learn more about Gmail label to folder migration in this article:
➡️ So before starting your email migration, choose whether you wish to skip duplicates (setup option 01) or migrate duplicates (setup option 02). Let’s look at the differences:
Migration Strategy 01:
Skip Duplicates
✅ This is MailJerry’s default setting and our recommendet strategy: An email will be copied into the folder in which it first appeared, preferring default folders (e.g. Inbox). All other copies are skipped.
Example:
| Email in Gmail | Email in MS 365 |
| Message stored in “Inbox” with label “Important” | Message only stored in “Inbox” |
| Message with label “Important” | Message stored in folder “Important” |
| Message with label “Important” and “New Business” | Message stored in folder “Important”, not in “New Business” |
✅ Advantages:
- No duplicates in your MS 365 mailbox.
- Smaller footprint, less storage consumption.
- No 1:1 clone of your Gmail mailbox.
- Emails with multiple labels only appear in one folder.
Advanced Settings to Skip Duplicates:
To use this strategy for your email migration, simply leave the advanced settings in their default state:
| Setting | Toggle | Comment |
| Migrate Duplicates | ❌ disabled | |
| Enable Cache | ❌ disabled | |
| Reduce Bandwidth | ❌ disabled | ⚠️ Keep this setting disabled, otherwise, you’ll get a 1:1 clone of your mailbox with duplicates. |
⚠️ When working with the Microsoft archive, please migrate your emails in date-filtered batches, giving your MS mailbox enough time in between runs to move your data to the archive. Do not perform a re-sync and don’t enable the backup mode! (Otherwise, you’ll receive duplicates.)
On the last run, only include emails you wish to store in your mailbox (not in your archive). For this migration, you can enable auto-sync and / or backup mode if required.
Migration Strategy 02:
Migrate Duplicates
By following this strategy, you can simulate the structure of your Gmail inbox and receive a 1:1 clone. This approach is only recommended if you want to replicate your Gmail label structure with IMAP folders, as it will create multiple copies of the same email.
⚠️ When opting for this strategy, we strongly recommend to disable IMAP for the “All Mail” label in Gmail (see tutorial).
Since Gmail automatically adds the “All Mail” label to every email, you’ll otherwise receive at least one duplicate for every email migrated.
Example:
| Email in Gmail | Email in MS 365 |
| Message stored in “Inbox” with label “Important” | Message stored in “Inbox”, duplicate stored in folder “Important” |
| Message with label “Important” | Message stored in folder “Important” |
| Message with label “Important” and “New Business” | Message stored in folder “Important”, duplicate stored in folder “New Business” |
✅ Advantages:
- No duplicates in your MS 365 mailbox.
- Smaller footprint, less storage consumption.
‼️ Downsides:
- Your new mailbox contains significantly more emails than your Gmail account (since duplicates are created).
- Much more storage space required (especially when migrating the “All Mail” label).
Advanced Settings to Migrate Duplicates:
To create a 1:1 clone of your Gmail label structure, configure the advanced settings as follows:
| Setting | Toggle | Comment |
| Migrate Duplicates | ✅ enabled | If you enabled “Reduce Bandwidth”, you can leave this option disabled. |
| Enable Cache | optional | By enabling the cache, MailJerry will know which emails were already migrated. So if you move or delete messages in the target mailbox, they will not be migrated again. When enabling the cache, make sure to disable IMAP for the “All Mail” label in Gmail to prevent the creation of unnecessary duplicates. |
| Reduce Bandwidth | optional | If your MS 365 mailbox is empty, enable this option as it significantly speeds up your migration. |
⚠️ Enable the cache when working with the Microsoft archive!
Otherwise, archived emails will be migrated again when performing a re-sync.
Bonus:Additional Setup Tips
After configuring the advanced settings, you’re nearly ready to start your migration. Before doing so, have a look at these additional setup tips:
Tip 01:
Don’t Ignore Gmail Bandwidth Limits
When migrating large Gmail mailboxes, please don’t enable the option “Ignore Gmail Bandwidth Limits”. For large-scale data migrations from Gmail to Office 365, we recommend to respect Gmails bandwidth limits. By doing so, your migration will take longer, but you won’t risk getting disconnected by Google.
Tip 02:
Consider Enabling the AutoSync or Backup Mode
➡️ Both Gmail and Microsoft 365 apply bandwidth limits to your accounts, which is why we recommend to enable the auto sync or backup mode (daily interval). If your migration exceeds the daily bandwidth limit of one of the two providers and your server connection is closed, the auto sync or backup mode will automatically resume your migration after the selected time, so you don’t have to restart it manually.
Learn more about the auto sync and backup mode for large mailbox migrations here.
⚠️ Don’t enable the auto sync when skipping duplicates and working with the Microsoft archive to prevent duplicates.
➡️ Once you’ve chosen your migration strategy and reviewed our bonus tips, it’s time to start your migration!
TroubleshootingAfter the Migration
When migrating large mailboxes from Gmail to MS 365, there are two common problems that may occur. So let’s have a look at how to resolve them:
Problem 01:
My Migration Keeps Breaking
When migrating very large mailboxes (50 GB and more), it may happen that you constantly exceed Gmails and/or Microsoft’s bandwidth limits and your IMAP connection to one or both servers is closed repeatedly. If this happens, consider migrating your data in date-filtered batches.
➡️ Migrating your data in date-filtered batches helps to keep your providers bandwidth limits in check and is especially convenient when working with the Microsoft’s archive feature.
Problem 02:
After the Migration, I Have Twice as Many Emails
When opting to migrate duplicates, your MS 365 mailbox can be twice as large as your Gmail inbox, even if you didn’t assign multiple labels to your messages. The reason for this lies within Gmails auto-labelling feature: Gmail automatically assigns the “All Mail” label to every email that is received in your account. This means that every email has at least two labels: “All Mail” and the label assigned by the user (e.g. “Inbox”).
➡️ To prevent these unnecessary duplicates, simply disable IMAP for the “All Mail” label in your Gmail account prior to the migration.
💡 Are you still encountering problems with your large Gmail to MS 365 migration?
If so, have a look at this list of troubleshooting tipps for large MS 365 migrations.
