Migrate 1000 Legacy Postfix/Dovecot IMAP Mailboxes to Microsoft 365

What IMAP Can and Cannot Move

Postfix handles email transport. Dovecot provides server-side IMAP access to stored messages. Migration tools only have access to data via IMAP on source servers. Here’s what can be migrated during a Legacy Email Migration to Microsoft 365, plus settings that need to be configured manually.

Item	Moves over IMAP
Emails and folder hierarchy	Yes
Attachments	Yes, with their messages
Read/unread status and standard flags	Yes
Contacts, calendars, tasks, notes	No, IMAP carries mail only
Rules, signatures, forwarding, aliases, quotas	No
Email account settings and passwords	No, these must be configured separately

Settings stored in Postfix or Dovecot configuration, like virtual aliases or sieve filters, have to be recreated in Microsoft 365 separately. Document them before you perform the first migration wave or miss important email routing and filtering configurations.

Prepare the Source and the Destination

A short checklist saves days later.

• Get the real account list. Running dovecot user ” on the Dovecot server will list all users, and it’s often different from the spreadsheet you’ve been maintaining for years.
• Create and license all 1000 Microsoft 365 mailboxes first. You can’t have a destination mailbox without a license to migrate data into it.
• Test IMAP Access over TLS from the migration machine, usually on port 993, or 143 with STARTTLS.
• Generate the CSV file containing email address, password, IMAP host, and port.
• Reduce the MX records’ TTL value a few days before cutover to begin the DNS propagation process.

Where Native IMAP Migration Gets Tight

Microsoft 365’s built-in IMAP migration in Exchange admin center uses a CSV file and IMAP endpoint to connect source mailboxes. At 1000 mailboxes, credential management & CSV preparation, plus progress monitoring, need to be done. IMAP migrations only transfer email content while leaving contacts, calendars, tasks, or mailbox rules intact. Microsoft’s limitations are 500,000 items/mailbox and 35 MB of maximum message size. Check mailbox item counts, message sizes, and authentication requirements before migrate IMAP mailbox to Microsoft 365 to avoid migration issues.

Limitations of the Conventional Method

When users migrate Postfix mailboxes to Microsoft 365, they may encounter some limitations.

• This method requires users to invest long hours to migrate 1000 mailboxes.
• Even the smallest typo in a CSV file will make your mailbox migration fail.
• The number of mailboxes that can be migrated at the same time depends on the type of migration you’re using.
• It’s possible that nested folders will not keep their hierarchy or names when being migrated.
• Users may receive limited error details when something fails.
• Users need a good understanding of IMAP and DNS fundamentals.

If you are planning to move mailboxes to Google Workspace instead, follow our guide on how to migrate IMAP email to Google Workspace.

A Wave Plan to Migrate 1000 Legacy Postfix/Dovecot IMAP Mailboxes

Migrating 1000 mailboxes in separate groups, or waves, rather than all at once is a better plan. This is because if there is a problem with any wave, it will be much easier to troubleshoot and isolate the issue.

Wave	Mailboxes	Purpose
Pilot	20	Test authentication, folder structure, and large attachments
Wave 1	300	Low-risk departments
Wave 2	300	Remaining departments
Wave 3	380	Largest and most sensitive mailboxes, once the process is proven

Postfix continues to deliver during the initial copy. Upon cutover, point MX to Microsoft 365 and perform a final delta pass since any messages delivered by Postfix before DNS propagation will still be received by the old server. Leave this server read-only for a few weeks post-migration.

Using DRS Softech IMAP Migration Tool for Batch Work

The DRS Softech IMAP Backup Tool offers both Single and Batch/CSV modes for managing IMAP accounts. Using the provided workflow, administrators can process mailbox data and migrate emails to the supported destinations, including Microsoft 365, if the selected workflow offers such a possibility. Moreover, in Batch mode, it is possible to prepare the list of required source accounts (email, password, IMAP server, port) and check if the authentication method, destination settings, and migration options are supported by the tool.

Workflow ke steps:

1. Download and install DRS Softech IMAP Backup Tool
2. Select the IMAP Migration option.
3. Choose Single Mode or Batch Mode authentication, depending on the number of accounts and the available workflow.
4. Load the mailbox data, preview the folder structure, and select the folder or item for migration.
5. Configure the Microsoft 365 destination and select the required sign-in authentication.
6. Apply the advanced filters, review the migration settings, and start Migration.

Single Mode is suitable for pilot users and those who need special treatment. Those who want to export IMAP mailbox to Microsoft 365 for a handful of executives can keep these accounts separated from the rest.

Key Features for a 1,000-Mailbox Project

Feature	How it helps here
Tree preview with folder-level counts	Gives a baseline to compare against after migration
Date filter and Advanced Mail Filter	Moves selected periods first, or targets Subject, To, From, CC, BCC, and Body
Skip Previously Migrated Data	Helps skip previously migrated data; it does not necessarily perform a final delta sync
Duplicate removal	Removes duplicates based on To, From, Subject, Body, and CC
All Reports and Active Tasks	Helps track processed, completed, skipped, and failed items

The migration tool recommends mailbox size limits as a step in the migration procedure. Still, migration time depends on mailbox size, network bandwidth, and servers’ performance, and plan wave windows around the slowest accounts.

Conclusion

Migrate 1000 legacy Postfix/Dovecot IMAP mailboxes to Microsoft 365: plan ahead, use a wave plan, and verify the migration. Manual migration requires a lot of attention and CSV file preparation, along with proper credential management and monitoring. The dedicated tool helps organize the migration of multiple accounts and provides you with migration progress monitoring. The Tool offers Batch/CSV, mailbox preview, filters, and reports. Use the free Demo to check how the tool works with test mailboxes before buying the service.

Frequently Asked Questions