Issue

During the PrePilot/Pilot, Autodiscover issues in the Outlook fat client caused login failures for approximately 10%–15% of migrated users on managed devices, which is expected to cause a significantly higher impact during the upcoming migration waves.

Recommendation

Background & Context

Root cause

Because the primary MX record still points to Gmail (a necessary setup during the coexistence period), Outlook Autodiscover behaves inconsistently during sign-in and may attempt to configure the user’s Gmail mailbox instead of the M365/Exchange Online mailbox.

Once the MX record is switched from Google to M365 after June 8, this underlying condition should be removed and the root cause is expected to be resolved.

Migration execution observations

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

• Approximately 10%–15% 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, despite several escalations and follow-up calls over 1 month.
  • Resolution ETA and feasibility is uncertain as of 06/06,meaning a fix willlikely not arrive before the first migration wave.

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 (Outlook web version still accessible).
• This would likely result in: High incident volume (300 to 400 tickets a week potentially) resulting in longer resolution times for other issues.
  • 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 package).
• The Autodiscover/login issue has been observed only on Managed devices (corporate-managed endpoints). Non-managed/BYOD scenarios hve not generated this issue so far.

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.
• Change Management need to revamp communication if not deploying fat client, this has an impact on the end user and may frustrate.

Options considered

Option 1) Use Outlook Fat Client

Option 2): Use Outlook Online

Option 3) Disable Autodiscover and User setup Outlook manually

Evaluation

Criteria	Option 1) Use Outlook Fat Client	Option 2): Use Outlook Online	Option 3) Disable Autodiscover and User setup Outlook manually
Technical Feasibility	Medium/Low for Day 1 – known Autodiscover/login issue affects ~10–15% 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	Medium – technically possible via policy/config, but requires clear, consistent manual configuration steps;
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)	High – all users must do manual setup; higher chance of user errors, delays, and frustration;  Allows PST and offline once configured
Support Impact	High – likely spike in incidents (profile creation, auth, Autodiscover troubleshooting) with longer handling time per ticket	Fewer/none Autodiscover-related incidents; Potential increase in incidents around PST migration/archives Last minute user guidance/adoption	Very high – large expected volume of “walk-me-through” tickets; requires strong floorwalking/hypercare and step-by-step guides
Operational Complexity	Low – requires Service Desk KBA, potentially device-by-device remediation	Medium – requires enforcing “no fat client” via M365 packaging/exclusion.  Significant last-minute change management adjustments necessary	High – must deploy policy to disable Autodiscover, Last minute communciations and instructions provided by change management, Requires coordination across waves
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	Higher indirect cost – significant time spent by users and support teams during each wave; may require additional hypercare resources

See also

The following section describes relevant documentation: