Osiris vs. MailStore Server
MailStore (Viersen, Germany; part of OpenText since 2019, branded "MailStore by OpenText" since version 26.2) is a genuinely German email-archiving vendor, which is worth stating plainly since it is the one product on this list actually headquartered here. It is an email archive first (the vendor's own documentation says so explicitly), not a full Microsoft 365 backup covering calendar, contacts, OneDrive or SharePoint.
| Fact | Source | Confidence |
|---|---|---|
| Self-hosted, Windows only. Once an installation passes 25 users, a full Windows Server edition is required. | System Requirements | Confirmed, official |
| Licensed per user with a perpetual right of use. A separate, time-limited Update & Support subscription (1, 2 or 3 years, Standard or Premium) is sold alongside it; the software keeps running after that subscription lapses. | Ordering and Licensing FAQ | Confirmed, official |
| Reactivating a lapsed Update & Support subscription is not a normal renewal: the vendor's own published terms price it as an "upgrade" at 70% of the current list price of the existing licences. | Ordering and Licensing FAQ | Confirmed, official |
| A starter package covers 5 users. | Ordering and Licensing FAQ | Confirmed, official |
| An IDW PS 880 software certification exists for Germany, Austria and Switzerland: a real, named German auditing-standard certificate, not a self-declared claim. Publicly downloadable certificate PDFs go up to version 23.4; the current certificate is available on request. | Rechtssicherheit (legal certainty) | Confirmed (certification exists); current version not independently re-verified this pass |
| Storage is file system or NAS. SQL/PostgreSQL-backed stores have taken no new installations since version 26.1 and are being phased out entirely by the end of 2026. No S3, Object Lock or WORM storage support is documented. | Storage Locations | Confirmed, official (WORM/S3 absence is a negative finding) |
| Export formats: EML/MSG, PST, or direct export into an M365/Exchange/Google/IMAP mailbox with folder structure preserved; signed exports can be verified with OpenSSL. The internal storage format itself is not openly specified. | Exporting Email | Confirmed, official (internal-format opacity is a negative finding) |
| A schedulable integrity check ("VerifyStore") exists; an automated end-to-end test-restore comparable to reading data back and diffing it is not documented. | Maintenance and Repair | Confirmed, official (negative finding on automated test-restore) |
| Not open source: proprietary commercial software owned by OpenText. | Absence checked across the pages above | Confirmed (negative finding) |
How Osiris differs
MailStore's perpetual per-user licence is structurally close to Osiris's own one-time-purchase model (both reject a pure subscription), but MailStore's 70%-of-list-price reactivation cost for a lapsed support subscription contrasts with Osiris, where all updates are included for as long as it is actively developed and there is no renewal to lapse (see pricing), and MailStore's Windows-only requirement and lack of S3/WORM/Object-Lock support are real constraints Osiris does not share (S3-compatible storage with Object Lock is a supported target from day one). MailStore's IDW PS 880 certification is a real, verifiable credential Osiris does not have yet, worth naming rather than glossing over; Osiris's own archive layer built for German GoBD requirements is still in development, see theroadmap.
All facts above from MailStore's own official documentation, checked 2026-09-23 and spot-re-checked 2026-09-24. Prices, editions and documented behaviour change. Verify current terms directly with MailStore before deciding. This page is not an endorsement of, or argument against, MailStore; it states what its own documentation says.