MailStore is an archiving tool, not a migration or conversion tool. While it can play a supporting role in a migration project, using it as the primary mechanism to move mail between servers will produce a poor result. The design choices that make MailStore a good archive are the same choices that make it unsuitable as a migration tool.

 Use a purpose-built migration tool

For Microsoft 365 and Exchange Online, use one of Microsoft's officially supported migration methods.

For IMAP-to-IMAP migrations, tools such as IMAPSYNC and Audriga are well suited to the job and preserve the details a migration needs to preserve.

 Why MailStore is a poor fit for migration

  • Mailbox metadata is not retained. Read/unread status and flags indicating whether a message was replied to or forwarded are lost on export.
  • Internal received dates are not preserved. MailStore does not retain the IMAP INTERNALDATE (or the Exchange equivalent). This value is not part of the message header but is what mail clients use to determine the true age of a message. A proper migration preserves it.
  • Folder structure is nested under an export folder. Exported messages are placed inside a "MailStore Export" folder that mirrors the user's original folder tree beneath it. Messages do not land back in their expected folders.
  • Archiving carries significant overhead. Archiving involves indexing, deduplication, compression, encryption, recovery record generation, and updates to multiple databases and the full-text index. A migration tool simply transfers messages from one server to another without any of that work.

 Where MailStore does help

MailStore can reduce the volume of mail that needs to be migrated. Archive mail from the old server first, then either delete older messages from the source before migration or archive everything and start with empty mailboxes on the new server. In both cases, users retain access to older messages through MailStore. Once the new platform is in place, MailStore can continue to archive from it to provide the ongoing benefits of archiving.