Status
Owner	James Kyndt
Stakeholders	Steering Committee

Decision:

Decision made by:

Date:

Online Meeting:

Issue

During the PrePilot/Pilot, Autodiscover issues in the Outlook fat client caused login failures for approximately 10%–20% of migrated users on managed devices, which is expected to cause a significantly higher impact during the upcoming migration waves.

Recommendation

Background & Context

Migration execution observations

During the PrePilot/Pilot, a recurring issue was identified affecting managed devices:

Approximately 10%–20% of migrated users experienced login / profile connection failures when using Outlook Desktop.
The symptoms are consistent with Autodiscover-related failures (e.g., Outlook cannot correctly locate mailbox settings or authenticate to the right endpoint post-migration).

Vendor dependency

A Microsoft support ticket is open, but:
- No confirmed fix or workaround has been provided yet.
- Resolution timing is uncertain, creating a risk that a fix will not arrive before Day 1.

Business impact observed / expected

If Outlook Desktop remains the Day‑1 standard and the issue persists:

A significant subset of users may be unable to access email/calendar via the desktop client at cutover.
This would likely result in:
- High incident volume and extended troubleshooting per user
- Increased Service Desk load and longer restoration times
- Productivity loss and reduced confidence in the migration

Assumption

Disabling / not deploying the Outlook fat client is feasible ahead of the Migration Waves by adjusting the M365 application packages (e.g., excluding Outlook Desktop from the standard deployment / upgrade package).
The Autodiscover/login issue has been observed only on Managed devices (corporate-managed endpoints). Non-managed/BYOD scenarios are not in scope for this specific issue unless later evidence shows otherwise.

Constraints / Impacts

PST limitations: Outlook on the Web does not support direct use of local PST files, which may generate additional incidents for users relying on PSTs for archives or operational folders. A mitigation (communication + guidance on alternatives) may be required.
Offline working limitation: Outlook on the Web provides no true offline mode, which impacts users who need email/calendar access during travel or in low/no connectivity situations. This may require user segmentation (e.g., critical offline users) and/or interim alternatives.

Options considered

Option 1) Use Outlook Fat Client

Option 2): Use Outlook Online

Evaluation

Criteria	Option 1) Use Outlook Fat Client	Option 2): Use Outlook Online
Technical Feasibility	Medium/Low for Day 1 – known Autodiscover/login issue affects ~10–20% of migrated users on managed devices; Microsoft fix pending/uncertain	High for Day 1 – bypasses Outlook profile/Autodiscover issues; generally stable access path if OWA/SSO/CA validated
User Impact	High risk of Day‑1 inability to access mailbox for impacted users; better for PST usage and offline work when it functions	Lower Day‑1 access risk for most users; no PST support and no offline working, may impact specific personas (travel/offline, PST-dependent)
Support Impact	High – likely spike in incidents (profile creation, auth, Autodiscover troubleshooting) with longer handling time per ticket	Medium – fewer Autodiscover-related incidents; potential increase in incidents around PST migration/archives and user guidance/adoption
Operational Complexity	High – requires troubleshooting playbooks, potentially device-by-device remediation; dependency on Microsoft resolution	Medium – requires enforcing “no fat client” via M365 packaging/exclusion; plus communication and handling exceptions for special cases
Cost	Potentially high indirect cost due to productivity loss and Service Desk load from 10–20% failures	Lower indirect cost (more predictable Day‑1 access); possible incremental effort for comms/training and PST/offline workaround handling

Description	Repository