Forum Discussion
Managing Large Shared Mailboxes in Exchange Online – Performance Strategies and Trade-offs
Hey everyone,
We’re managing very large shared mailboxes (>30 GB) in Exchange Online. These mailboxes are accessed by multiple users, with constant activity — dozens of emails being read, moved, flagged or replied to per minute.
Now:
- If we cache the shared mailbox in Outlook, the .ost file grows massively (10–20+ GB), which leads to local performance issues and even sync glitches.
- If we don’t cache, then Outlook has to fetch everything live from Exchange Online, which introduces delays and makes search slower or inconsistent.
=> So basically, performance sucks either way.
What we’ve learned so far:
- Shared mailboxes are treated like secondary mailboxes in Outlook, meaning:
- They sync slower than the primary mailbox.
- Push notifications from Exchange are limited or absent.
- Outlook often polls instead of getting real-time updates.
- Microsoft applies throttling policies per mailbox and tenant, which affects shared mailboxes with many concurrent users.
- OWA (Outlook Web Access), and the new Outlook app (One Outlook), use a persistent connection (WebSockets / streaming), allowing true real-time updates — no polling, no .ost reliance, no lag.
- The classic Outlook (Win32) client relies on MAPI and old-style caching behavior, which makes it less ideal for fast-paced shared mailbox environments.
What we’re now considering:
- Should we move high-activity shared mailboxes to be accessed via OWA or the new Outlook app, where real-time sync is better?
- Should we split large shared mailboxes into smaller functional ones (e.g. support@, sales@, escalations@) to reduce contention?
- Should we still use caching, but limit it to Inbox + Sent Items and 3–6 months, and invest in better client hardware (faster SSDs, 16–32GB RAM)?
- Is it worth mapping shared mailboxes as full secondary accounts rather than traditional shared folders, to improve sync reliability (with the right licensing)?
- Or should we just give users personal mailboxes instead, and use distribution groups or automation for collaboration?
1 Reply
OWA is the easiest solution, assuming all the features you need are supported therein. Use the Open another mailbox functionality (assuming Full access has been granted) or just use direct links to open the shared mailbox (i.e. https://outlook.office.com/mail/email address removed for privacy reasons/). The new Outlook still has to add proper support for some shared mailbox scenarios, but it is also an option to consider.
Second to that is reducing the mailbox size, either by cleaning up older items or enabling archiving (subject to license availability). Configuring the shared mailbox as account in Outlook can also help in this regard, as it will expose the cache slider, and even more granular sync controls on the folder level. You do not need a license for that btw, just full access permissions.
Whether DLs or Microsoft 365 Groups are an appropriate solution I cannot tell, you have to decide on that based on the functionalities you need and the use case scenarios.
