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Status

Status
colourBlueGreen
titlePending Decisiondecided

Owner

Jean-Baptist Lanneluc

Stakeholders

Steering Committee


Info

Decision:  Option 2: Outlook: 3 months data & 50% bandwidth download; OneDrive: 50% bandwidth upload

Decision made by:  IT Steering Committee

Date:   

Online Meeting:  MS Transformation - IT SteerCo #3

Issue

Potential network saturation on Day 1 post‑migration (GWS → M365) due to Outlook Fat Client initial sync/download of large volumes of emails and attachments.

...

  • This will not impact all users, only E5 with corporate device and fat client of Outlook,
  • Outlook is configured to download mailbox content including attachments,
  • Even if Outlook is set to only download 1 month of historical emails locally, the other emails will still be found via the research feature on the fat client (that relies on the web) if the employee is online.
  • Network capacity is not dimensioned for a “mass concurrent bulk download” event, so congestion may occur and impact other business-critical traffic, especially on certain sites that already face network challenges.

...

  • Outlook becomes slow/unresponsive during initial caching (long “Updating mailbox” / “Trying to connect”), delayed send/receive, attachments opening slowly.
  • Outlook search experience for old items might require online connection and will be slower to load and display these items.
  • OneDrive sync backlogs: long longer time until Desktop/Documents/Pictures are fully available in the cloud; users may see “sync pending” and missing files on other devices.
  • More file conflicts/duplicates if users edit files while large sync is still in progress.

...

EvaluationOption 1: Outlook: 1 month data & 50% bandwidth download; OneDrive: 50% bandwidth uploadOption 2: Outlook: 3 months data & 50% bandwidth download; OneDrive: 50% bandwidth uploadOption 3: Outlook: 1 year data & 50% bandwidth download; OneDrive: 50% bandwidth upload
Technical Feasibility(plus) OK(plus) OK

(plus) OK

User Impact

(plus) Limited network congestion on site for a short time (1 hour at most)

(minus) Users will need internet access to browse their emails beyond 1 month

yellow circle Limited network congestion on site for several hours (up to 3 hours for many sites)

(minus) Users will need internet access to browse their emails beyond 3 months

(minus) Limited network congestion on site for a full day for many sites.

(plus) Once downloaded, 1 year mails accessible from outlook without internet access.

Support Impact

yellow circle Medium: fewer “network is slow” incidents, but more end-user questions like “I can’t find older emails in Outlook” / “Search doesn’t show old mail” (they’ll need to use Outlook Online be online to access >1 month which may display and open slower).

yellow circle Some tickets for how to access archives/older mail and user guidance.

(plus) Low–Medium: good balance; fewer “older mail missing” tickets than Option 1.

yellow circle Some performance/sync complaints may remain in the first hours, but generally manageable.

(minus) Medium–High: more Day‑1 slowness risk (longer caching), leading to more tickets (“Outlook stuck/slow”, “can’t send/receive”, Teams call quality issues due to congestion).

(plus) Fewer “older email missing” questions, but more performance/network-related incidents.

Operational Complexity

(minus) More change-management effort due to behavioral change for users.

yellow circle Low–Medium: same type of controls as Option 1, but less user disruption and fewer exceptions.

(plus) Standard monitoring and comms.

(minus) Medium–High: higher need for Day‑1 command center, active network monitoring, potential reactive throttling, and site-by-site troubleshooting.

(minus) More coordination with network team.

Cost

(plus) Low direct cost (configuration/policy + comms).

(plus) Low direct cost.

(plus) Low direct cost for configuration.

...