Third-Party Email Archive Migration to Microsoft 365

Mimecast, Veritas Enterprise Vault, Google Vault, Barracuda, and Proofpoint archives are the common cases; the approach — assess, plan the extraction, pilot, migrate in logged batches, reconcile — applies to other archivers too. Each platform has a genuinely different exit path, which is why the assessment names the method and tooling for yours specifically before anything is approved.

Typically as EML or PST exports, either self-service in batches or through Mimecast's own paid export service for larger volumes — the vendor route is priced per gigabyte on your Mimecast contract and has a queue, which we build into the schedule. The extraction plan states which route fits your volume and itemizes the vendor cost separately, so you see it as Mimecast's line, not hidden in ours.

A written, batch-level record from export to verification: what was extracted, when, in what format, what was ingested where, the item counts at each step, and the verification result. Combined with the final reconciliation report and the exception register, it gives your legal team a defensible account of the move — the thing that is impossible to reconstruct after the old platform is switched off.

They are inventoried, mapped to Microsoft Purview equivalents — litigation hold or eDiscovery holds for custodians, retention policies and labels for rules — and implemented in the destination before data lands. Nothing migrates a hold automatically, so this is deliberate, documented work your compliance owner signs off. The source stays readable until reconciliation is complete, so there is no window where held data exists nowhere enforceable.

Journaling captures one copy of every message with an envelope wrapper recording the true recipients, including BCC and distribution-list members. To make that meaningful in Microsoft 365, envelope data has to be read and each message attributed to the right custodians — decisions with real compliance weight, especially for leavers and lists. We document the mapping rules and get them approved rather than letting a tool guess.

Wherever it is most usable under your compliance rules — usually each user's In-Place Archive, so old mail is searchable in Outlook without swelling the primary mailbox, with leavers going to inactive or shared mailboxes. Once in Microsoft 365, everything is subject to Purview retention and searchable through eDiscovery. The mapping design puts the destination for every custodian in writing before migration.

In-Place Archives require Exchange Online Plan 2 or the Exchange Online Archiving add-on, and hold and retention features depend on your plan level. Rather than list every SKU permutation here, the assessment states exactly what your destination design requires against what you already own — often the licensing you have is already enough.

Their data is often the main reason the archive exists, so leavers get an explicit decision, not a default: retained in inactive mailboxes, consolidated into shared mailboxes, or excluded with your sign-off. The assessment lists every leaver archive it finds, and the mapping design records what you chose for each.

Because the bottleneck is the source. Export services have queues, APIs are throttled, and journal archives need mapping before ingestion — none of which is fixed by us working harder. The typical engagement runs about four weeks; the assessment gives you a schedule grounded in your platform's real export behavior, and the batch log shows progress continuously rather than asking you to trust the plan.

Reconciliation, not reassurance: per-batch counts in the chain-of-custody log, a final source-to-destination comparison, and an exception register in which every skipped or failed item has a stated reason and disposition. You sign the reconciliation report before we call the migration done — and before anyone talks about decommissioning the source.

Yes, and you should — the source stays live and readable until reconciliation is signed off. That overlap is a deliberate safety property: it preserves your search capability, keeps held data enforceable somewhere at every moment, and gives the reconciliation something to reconcile against. The cost of a few extra weeks of the old contract is the cheapest insurance in the project.

No. In-Place Archive Migration and the tenant-to-tenant variant move Exchange archive mailboxes between Exchange environments and Microsoft 365 tenants using Microsoft's native migration paths. This service handles third-party archive platforms, where the hard work is getting data out of a proprietary store and rebuilding compliance settings — a different problem with different tooling and economics.

Usually yes, with sequencing: custodians and data attached to active matters are identified in the assessment, their holds are re-created in Purview first, and where a matter is genuinely mid-production your legal team may keep that slice in the source until the case allows. Ongoing search and case work over the migrated data is its own service — Purview eDiscovery Search Assistance — and we hand over cleanly to it.