Lowest UID 0 is less than 1 Error

Error: Lowest UID 0 is less than 1.

This error is caused by a well-known bug in the IMAP implementation used by your email server.

Internally, each IMAP message is assigned a unique ID (UID). MigrationWiz retrieves UIDs from the Source server as a way to implement fast data retrieval. As per RFC 3501 (which defines how the IMAP protocol works), all UIDs must be greater or equal to 1. In the formal specification (paragraph 9), a UID is defined as an "nz-number", which itself is defined as a non-zero value.

However, due to invalid implementations, some servers may return a lowest UID value of 0. For example, we've seen situations where sending the following request: "UID FETCH 1:1" results in the following response: "(UID 0)". This is obviously incorrect since the requested range was 1-1.


Upgrade Your IMAP Server

Consider upgrading your IMAP server to the latest version.

Disable Strict Validation of UIDs

You may disable validation of UIDs:

  1. Sign in to your MigrationWiz account
  2. Click on Mailbox Migration.
  3. Click on Migrate Mailbox.
  4. Select your Mailbox Connector.
  5. Click on View Mailboxes.
  6. Select your Mailbox.
  7. Click on Edit Mailbox.
  8. Click on Show Advanced Options.
  9. In the Support Only options, enter AllowZeroUid=1.
  10. Click on Save​.

Note: You can also set the same Advanced Option on the project. In this case, the setting will apply to all mailboxes in this project.

By setting this option, MigrationWiz will assume that the lowest UID is 1. As a result, it is possible that a message with a UID of zero in each folder will not be migrated.

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