Microsoft Purview eDiscovery vs Relativity

Microsoft Purview eDiscovery and Relativity get compared constantly, and the comparison is usually framed wrong. They do not sit on the same axis. Purview identifies, preserves, collects, and culls the data Microsoft already holds, then stops before production. Relativity starts at processing and review and carries the matter through Bates numbering, redaction, privilege logs, and production. One is a collection layer you already own; the other is a review platform you buy per matter.

So the buyer question is not “which one.” It is where the handoff happens, and who pays for the gate on either side of it. That answer changed in the last year, because both vendors shipped the bridge.

The question the E5 line hides

The recurring version of this question is “Microsoft 365 E5 already includes eDiscovery, so when is paying for Relativity justified?” The premise is half true, and the half that is false is expensive.

What E3 buys is search, KeyQL queries, statistics, holds, search-and-delete, and export of search results. What requires premium licensing is everything that makes Purview a review platform: review sets, advanced indexing, analytics such as near-duplicate detection and email threading, OCR, decryption, conversation threading, and tagging. Purview’s premium gate is also licensed against the person whose data you analyze, not the person doing the analysis — a fully licensed legal team on an E3 tenant can open the case, run the search, place the hold, and still be unable to build a review set over the custodians who matter.

That distinction decides the whole comparison, for a reason most feature grids miss: Relativity’s own import bridge lands on the premium side of it.

Where Purview wins

• Preservation in place, at no marginal data cost. Holds attach to Exchange Online, SharePoint, OneDrive, Teams, Microsoft 365 Groups, and Viva Engage without copying anything out. For employment claims, internal investigations, and subpoena response that resolve inside Microsoft 365, nothing leaves the tenant and no per-GB meter starts.
• The cost curve is flat in data volume. You pay per person per month whether the matter is 4 GB or 400 GB. On a high-volume, low-headcount matter this is the cheapest defensible collection in the market, because you have already bought it.
• Targeted premium licensing beats a tenant-wide upgrade. On a 2,000-seat E3 tenant with 60 recurring custodians, the Microsoft Purview Suite add-on at $12 per user per month costs roughly $8,600 a year. Moving the whole tenant to E5 for the same capability is the $21 per user per month delta — about $504,000 a year. Same outcome, 58x the invoice.
• Early case assessment before anyone commits. Statistics and samples inside a search tell you the volume you are about to hand a review platform, which is the number that sets the Relativity quote.

Where Relativity wins

• It produces; Purview does not. No Bates numbering, no redaction tooling, no privilege log generation exists in Purview. Its attorney-client privilege detection flags likely-privileged documents against an attorney list you supply, and Microsoft’s own documentation states it does not replace privilege review. When a matter reaches production, the export leaves.
• Cost is flat in headcount, not just data. Relativity eliminated per-seat user fees in 2022 in favor of a bundled data fee. Staffing 40 contract reviewers onto a matter adds nothing to the license line. On Purview, every additional custodian under premium review adds one.
• AI is bundled, not sold as an add-on. RelativityOne carries aiR for Review, aiR for Privilege, aiR for Case Strategy, and aiR for Data Breach Response at no additional cost. That is a recent and load-bearing change: on 9 October 2025, effective 1 November, Relativity replaced document-volume metering for aiR with all-inclusive per-GB pricing, and aiR for Case Strategy moved into standard RelativityOne packaging in 2026. Any comparison still describing aiR as a per-document add-on is describing last year’s order form. Bundled is not the same as uncapped, though: Relativity still defines an aiR Unit as the extracted text of one document and keeps its thresholds and included-products table behind a Community login, so get your allowance written into the order form rather than assuming the per-GB fee covers everything.
• Non-Microsoft data has a path. Slack, Bloomberg, mobile archives, and forensic device images reach Purview only through paid third-party connectors that rewrite items into Exchange mailboxes as email, and device collections have no path at all. The moment a custodian used a personal device or the matter touches a non-Microsoft system, this decides it.
• It is the lingua franca when outside counsel runs the matter. If the data already lives there, moving it out to save hosting fees is a false economy.

Pricing reality

The two are quoted in units that do not convert. Purview is per user per month and recurring forever; Relativity is per gigabyte and scoped to the matter.

Microsoft publishes its side: Microsoft 365 E3 at $39 per user per month, E5 at $60, the Purview Suite add-on at $12 on top of E1 or E3, and pay-as-you-go at $20 per GB per month for non-Microsoft 365 AI application data added to a review set or export.

Relativity publishes nothing. Its pricing page offers pay-as-you-go or one- and three-year commitments and a Request Pricing form, with Relativity Contracts, Relativity FOIA, and Relativity Legal Hold quoted as separate modules. Reseller and consultant reporting current as of August 2026 puts RelativityOne hosting at roughly $15–30 per GB per month and one-time processing at $25–75 per GB, with volume commitments pulling toward the low end. Treat those as a negotiating anchor, not a rate card — and discount any source quoting Relativity per-user rates, because the vendor’s own model has not had a seat fee since 2022.

Worked example, using the estimate band. A 200 GB post-cull matter hosted for twelve months runs about $36,000–$72,000 in hosting plus $5,000–$15,000 in processing — call it $41,000–$87,000, with aiR included. Preserving and collecting the same matter across 60 custodians on Purview Suite costs about $8,600 for the year. Purview is roughly five to ten times cheaper for the collect-and-cull layer and cannot produce a single Bates-stamped page.

The path most buyers do not price

Once you accept the handoff, there are three routes, and the middle one carries a licensing tax nobody quotes.

1. Purview alone. Search, hold, export. Stops before production.
2. Purview → Purview Sync → RelativityOne. Relativity’s Purview Sync went GA in December 2025 and automates the export-stage-FTP-handoff cycle, converting HTML-formatted Teams and Copilot messages to RSMF, organizing by custodian, and overlaying metadata. But it imports review sets — and review sets are the premium-gated artifact. Purview Sync requires eDiscovery Premium access and an eDiscovery Manager or Administrator role, so this route pays Microsoft’s premium gate on every custodian and Relativity’s per-GB fee. Relativity also recommends a ceiling of 100 GB and 25,000 files per run, well under Purview’s own 500 GB export cap, so it is a mid-size-matter tool.
3. Relativity Collect straight from Microsoft 365. Collect has purpose-built connectors for Exchange, OneDrive, SharePoint, and Teams across commercial, GCC, and GCC High tenants. It never builds a review set, so it never trips the premium gate. On an E3 tenant, this route holds in Purview and collects with Relativity — and the premium licensing spend goes to zero.

Route 3 is the one to price first if Relativity is doing the review anyway. The Purview premium tier earns its money when you review in Purview; paying for it to feed a platform that also collects natively is buying the same capability twice.

Verdict

• Pick Purview when the evidence lives entirely in Microsoft 365, collected volume stays in the low hundreds of GB, the matter resolves without production — internal investigations, employment claims, regulator response, early case assessment — and preserving in place beats moving data out.
• Pick Relativity when the matter will produce; when data sits outside Microsoft 365 or on devices; when reviewer headcount is large relative to custodian count; when outside counsel or a service provider already runs it; or when you want AI review without a per-document meter.
• Use both — and this is the common answer for M365-heavy litigation, not a compromise. Hold and cull in Purview, review and produce in Relativity. Choose the bridge by volume: Purview Sync under 100 GB, Relativity Collect above it or whenever you want to skip the premium gate.
• Pick neither when the matter is a handful of custodians and a few gigabytes with no production in sight. A targeted search and export at E3 with outside counsel reviewing in whatever they run is cheaper than either answer here.

Default pick when you genuinely cannot decide: Purview first, Relativity when the matter escalates. The cannot-decide case is one where you do not yet know if the matter will produce, and Purview is already paid for while Relativity’s per-GB clock starts on ingestion. Collect and cull in place, then hand over a culled set instead of a raw one — the volume you avoid hosting is the largest single lever on the Relativity invoice.

Two guards to carry in. First, run notices somewhere neither of these does well. Purview’s custodian communications and acknowledgment tracking belonged to the classic experience that Microsoft retired on 31 August 2025; the current workflow is trigger event, case, search, hold, review, export, with no notice library. Relativity Legal Hold is a separately quoted module on Relativity’s pricing page, so it is a third line item, not a freebie — price it or run notices in Exterro and keep the acknowledgment chain documented apart from the system hold.

Second, export before you close anything. Purview’s pay-as-you-go meter keeps running on closed cases — only deleting the case stops it — and case data does not survive a tenant move: cases created before an Advanced Data Residency migration are unreachable from the destination tenant, with no in-place migration path. Make export-then-delete an explicit matter-closure step.

Related reading: the EDRM model for where each tool sits in the lifecycle, the legal hold process for what has to happen around the system hold, and Everlaw vs Relativity if the review platform itself is still open.