Below are common questions about running a migration with the data migration service.
Before you migrate data
Is there a maximum number of users I can migrate?In most cases, the data migration service works best when migrating a group of 100 users or fewer. You can migrate more users at once, but you might see a drop in performance.
For other migration options, go to the Google Workspace migration product matrix.
Run the migration after you add the Google Workspace domain. For details, go to Set up your new Google Workspace account.
Not at the same time. The data migration service won't migrate from multiple source accounts to a single target account. Migrating data to the same target account when a migration is already running doesn't add additional data to the target account. It cancels the first migration.
If you want to migrate from multiple source accounts, let the first migration complete before beginning a second migration. The second migration removes the other (completed) migration from the list of active migrations, but this won't affect any active migrations or mappings.
Example—Migrating user accounts
You're migrating data from source-user1 to target-user1. Then, you add another migration from source-user2 to target-user1. Target-user1 ends up with data from source-user2 and only partial data from source-user1 (rather than data from both source accounts). The second migration cancels the first migration.
Examples—Migrating calendar data
- You set up an identity mapping CSV file where multiple source identities are mapped to a single target identity. After a migration, only one of the mappings applies and the others are discarded. There are no errors or warnings that this occurred.
- You set up an identity mapping CSV file and map source-identity1 to target-identity1. Then, when you are setting up your migration, you click Add user and map source-identity2 to target-identity1. During a migration, source-identity2 is mapped to target-identity1 and the identity mapping is discarded without errors or warnings.
If source-identity1 is a user, events that have source-identity1 as an attendee or organizer don't transfer to the target user. The target user instead becomes the organizer or attendee of events that have source-identity2 as the organizer or attendee.
Planning a migration
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How long will a migration take?The duration of a migration depends on several factors, such as:
- The number of users you’re migrating.
- The migration start date you select—the longer the time frame, the longer a migration takes.
- The number and size of items being migrated.
If you’re not migrating from a Google Workspace or Gmail account, you also need to consider:
- The performance of the source server you’re migrating from.
- The network's data transfer rate.
- Latency between Google and the source server.
No. The data migration service migrates email or calendar events from the start date entered to the current day, beginning with the most recently dated events or messages. For details, go to Exclude old email messages or calendar events.
Note: You cannot change the migration settings (including the start date) once the migration has started.
No, resellers can't migrate data for their customer accounts.
During a migration
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How do I pause, cancel, or exit a migration?When you use the data migration service, you can pause, restart, cancel, or exit a migration. For details, go to Pause, cancel, or exit a migration.
To change the migration settings during a running migration, you must first cancel the migration. For details, go to Cancel a migration to edit the settings.
In some cases, the status bar doesn’t progress as expected and stalls on a percentage (usually 0% or 99%). This situation can occur for the following reasons:
- The data migration service is processing email messages that already exist in the target mailbox (for example, a message that exists in more than one folder in the source account) and is applying labels. The progress indicated by the status bar doesn't change during this task. When the labels are applied, the migration completes and the status changes to 100%.
- The number of items successfully migrated exceeds the total items discovered. The status bar reports 99% until the migration completes. This process might take some time.
- You have started the migration multiple times. The status bar doesn't update and, if you point to the bar, you get a "Mailbox estimation failed" message with the percentage stuck at 0%. Even though the status bar doesn't update, the migration hasn't stalled. For troubleshooting, you should monitor the migration status and note whether any messages fail to migrate.
Tip: You can check the metrics of a migration by pointing to the status bar. If the migration is proceeding, the Items successfully migrated or Items that failed to migrate numbers increase.
When the migration finishes, Complete appears in the Status column for a user in the Migrations table. To view metrics about a migration, point to Complete in the Status column. A tooltip displays the following statistics:
- Items successfully migrated
- Items that failed to migrate
- Total items discovered
Another way to verify if a migration completed successfully is to check the migration report for errors. For details, go to Monitor a migration.
Note: When the migration is complete, you should exit the migration. A migration report is sent to all super administrators of the account and the migration role account. For details, go to Exit a completed migration.
Migrating email data
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Can I migrate IMAP email folders that are shared or public?The data migration service does not support migrating shared or public IMAP folders.
By default, Google gives you 15 GB of storage space for everything associated with your Google Account. Your storage limit is shared across Google Drive, Gmail, and Google Photos.
You cannot import more data into Gmail than your existing storage space allotment. Messages and attachments, including items in your Spam and Trash folders, count toward your storage limit.
Important: When your account reaches its storage limit, or if you downgrade your account and your files now exceed your storage space, you cannot send or receive messages in Gmail.
For details on storage space, go to Manage files in your Google Drive storage.
Messages must be RFC-compliant to be migrated with the data migration service. If they aren't, you’ll receive the error message “Our system has detected that this message is not RFC 2822 compliant" or the message might be migrated with the wrong date.
Here are some common reasons why messages aren't RFC-compliant:
- The From field can’t be empty and must only have a single address.
- The domain in the From field can't be malformed.
- The Date field must have 4 digits for the year, not 2.
- The time of day field must be in the range 00:00:00 through 23:59:60.
If you can edit the message in the source mailbox, you might be able to correct the message headers and remigrate. If not, you should migrate the messages some other way (for example, by forwarding them to the new account using your email client). For information about correcting the messages, contact your source account provider.
The data migration service does not remove data from the source mailbox, nor does it remove the original source mailbox. All eligible data is copied during a migration, not moved or deleted, and the data in your source mailbox remains after a migration.
Note: The data migration service doesn't migrate messages, including attachments, larger than 25 MB. For details, go to Send attachments with your Gmail message.
Yes, you can choose a custom start date to specify the entire period on your source account. If you have many years or multiple gigabytes of data, we don't recommend migrating all email messages in a single migration. It takes a long time to complete a migration of this size. For details on custom start dates, go to Exclude old email messages or calendar events.
If you’re migrating from Microsoft Exchange or Exchange Online (Microsoft 365), any new messages delivered into the source account during the migration are transferred until you exit the migration. If you’re migrating from any other platform, new messages delivered to the source account during the migration aren't migrated. For more details, go to How do I migrate newly received emails?
Users can set up automatic forwarding on their source account to ensure any new email messages are sent to their new Gmail account. Consult the documentation for your source account for instructions on how to do this. If you’ve migrated from Gmail or another Google Workspace account, go to Automatically forward Gmail messages to another account.
If your MX records aren’t pointing to Google during the migration period, you can create automatic forwarding to a test domain alias. For details, go to Legacy server as primary server.
Existing labels are imported from the source account along with email messages. However, the data migration service does not apply a new label to indicate that a message was migrated from another account.
No. You can only migrate email messages to a user’s Gmail account. It’s not possible to migrate email data to Groups.
Yes. Using the data migration service doesn't require turning off 2-Step Verification or allowing access to less secure apps. The recommended approach depends on how many users you intend to migrate:
- If you’re migrating multiple users, create an App Password in the source accounts. Use the App Password instead of their regular password when adding the source account and follow the steps for migrating from Google Workspace. For details, go to Sign in with App Passwords and Migrate from Google Workspace.
- If you’re migrating a single user, follow the steps for migrating data from Gmail. For details, go to Set up & migrate from Gmail.
You don’t need to take any additional steps if the target account uses SSO or 2-Step Verification. The authentication methods used by the target account don't affect the migration process.
If users leave your organization but you want to continue retaining or holding their data with Google Vault, we recommend you assign the users Archived User licenses rather than import their data using the data migration service. For details, go to Preserve data for users who leave your organization.
Rerunning a migration
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If I rerun a migration, will email messages be duplicated?No. If you exit and rerun a migration, messages that have already been migrated won't be duplicated.
The data migration service checks if each message is already in the target mailbox before trying to migrate it. It skips messages that are already there.
No. If you exit and rerun a migration, any calendar events or secondary calendars that are already migrated won't be duplicated.
However, if a user changes the name of a secondary calendar between migrations, the calendar is duplicated when it's remigrated.
After a migration
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Once I migrate data, how do I migrate data from an earlier start date?If you previously migrated data from a specific start date, and later decide you want to migrate data from an earlier start date, you must exit the migration and start again using the earlier date. The data migration service checks every message before migrating it, identifies duplicate messages, and eliminates them. It does this check regardless of how the migration is run, or how the message was previously inserted into the account. For details, go to Specify a migration start date.
If you need to fetch emails received on the source mailbox after the migration completes, you must start another data migration. For more details, go to What happens to email received in the source account when a migration is running?
There’s no option to reverse or roll back a completed migration. If you need to complete another migration, you can delete all users and start again.
Migrated messages might show the migration time and date, rather than the time and date of the original message.
This most likely occurred because the date header of the original message wasn't compliant with RFC 5322. If a message has a date header that isn't formatted correctly, Gmail applies the migration time and date to the message.
If you see more items in the source mailbox or the Total items discovered count than are migrated, it might be because:
- Conversation view is turned on in the source account. If so, Gmail groups messages into a thread and the number of total messages is lowered. To display the total number of messages in All Mail, turn off Conversation view in the Gmail source account. For details, go to Group emails into conversations.
- There's a difference between Exchange and IMAP mail server folders and Gmail labels. In Exchange and on IMAP mail servers, when you assign a message to more than one folder, the message is duplicated. Following a migration, the duplications are removed in Gmail and different labels are applied instead.
- Exclusions have been applied to migrated content from Gmail or Google Workspace, such as skipping Junk messages or migrating data for a specific timeframe. The Total items discovered count includes all items in the All Mail view, regardless of any exclusions that apply to the migration.
To verify that all messages are migrated, check the Email domain migration report. For details, go to Understand migration reports.
If you think you’re missing messages, check whether the message is:
- Bigger than 25 MB (including attachments). You can’t migrate messages bigger than 25 MB.
- Blocked due to an attachment to the message. Gmail blocks certain types of attachments, such as executable files. For details, go to File types blocked in Gmail.
- In a folder or within a date range that’s part of the migration.
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