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.

Recommendation

Option 21: Outlook: 3 months 1 month data & 50% bandwidth download; OneDrive: 50% bandwidth upload

There are 2 Two actions need to be taken:
- Optimize applications to reduce the amount of data
- Apply bandwidth limitations to these applications for Day 1

The 2 applications mentioned are mostly OneDrive and Outlook.
- On Day 1, Outlook downloads 1 year of emails and attatchments locally
- While OneDrive uploads the content of the folders "Documents", "Desktop" and "Images" to OneDrive.

Background & Context

On the first business day after cutover of each Wave, many users will perform an initial synchronization of their mailboxes in Outlook fat client.

With the current setup this can trigger massive simultaneous On Day 1, Outlook downloads 1 year of emails and attatchments locally, which can generate massive downloads of historical emails and attachments from M365.

Similarly, OneDrive synchronization will consume some bandwith mainly to upload files from the computer main folders into Onedrive.

OneDrive uploads the content of the folders "Documents", "Desktop" and "Images" to OneDrive.

Assumptions

• 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.

Constraints

• Users still need to access these appliations for their business requirements, so the bandwidth limitation needs to be reasonable.

...

• 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.

...

Option 3: Outlook: 1 year data & 50% bandwidth download; OneDrive: 50% bandwidth upload

Evaluation

Evaluation	Option 1: Outlook: 1 month data & 50% bandwidth download; OneDrive: 50% bandwidth upload	Option 2: Outlook: 3 months data & 50% bandwidth download; OneDrive: 50% bandwidth upload	Option 3: Outlook: 1 year data & 50% bandwidth download; OneDrive: 50% bandwidth upload
Technical Feasibility	OK	OK	OK
User Impact	Limited network congestion on site for a short time (1 hour at most) Users will need internet access to browse their emails beyond 1 month	Limited network congestion on site for several hours (up to 3 hours for many sites) Users will need internet access to browse their emails beyond 3 months	Limited network congestion on site for a full day for many sites. Once downloaded, 1 year mails accessible from outlook without internet access.
Support Impact	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).  Some tickets for how to access archives/older mail and user guidance.	Low–Medium: good balance; fewer “older mail missing” tickets than Option 1.  Some performance/sync complaints may remain in the first hours, but generally manageable.	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). Fewer “older email missing” questions, but more performance/network-related incidents.
Operational Complexity	More change-management effort due to behavioral change for users.	Low–Medium: same type of controls as Option 1, but less user disruption and fewer exceptions. Standard monitoring and comms.	Medium–High: higher need for Day‑1 command center, active network monitoring, potential reactive throttling, and site-by-site troubleshooting. More coordination with network team.
Cost	Low direct cost (configuration/policy + comms).	Low direct cost.	Low direct cost for configuration.

See also

The following section describes relevant documentation: