Microsoft 365 slow email? How to fix it without pulling your hair out
Slow email is one of those small-but-crushing problems. It doesn’t make the headlines, but it eats minutes every day, frustrates clients, and chips away at staff morale. For UK businesses with 10–200 people, a lagging Microsoft 365 inbox can feel like a permanent traffic jam on a busy commuter line.
Why speed matters more than you think
Slow email affects more than just patience. Quotes get delayed, invoices take longer to send, and people switch to clunky workarounds — uploads to personal drives, WhatsApp, or multiple copies of the same document. Those workarounds cost time, create compliance headaches (hello, GDPR), and reduce confidence in your operations. Fixing the cause of slow email usually pays back quickly in reduced back-and-forth and fewer staff interruptions.
Common causes I see in UK businesses
From running support across small firms in the Midlands to dealing with practices in London, a few causes turn up again and again:
- Local network issues: A dodgy router, slow ISP route, or overloaded Wi‑Fi can make everything feel slow even when Microsoft’s cloud is fine.
- Outlook client problems: Old versions, corrupt cache files (OST), or too many add‑ins cause delays on users’ machines.
- Large mailboxes and attachments: Big PST/OST files or staff keeping endless attachments in folders slow search and sync.
- Rules, shared mailboxes and third‑party connectors: Complex inbox rules or poorly configured connectors can hold up delivery.
- Throttling or sync limits: Heavy automated processes or mass migrations can trigger temporary limits in Exchange Online.
Practical, business-focused fixes (no IT-speak)
Here are steps to try in a reasonable order. They’re written for managers who want results without wading into deep technical detail.
1. Check the obvious first
Ask someone to open Outlook Web App (Outlook in a browser). If mail is quick there, the cloud is fine and the issue is local to the user’s device or network. If web mail is slow too, it’s probably an Office 365 service issue or something on your network.
2. Test the network
Try a wired connection or a mobile hotspot from a phone. If performance improves, look at the office router, switches, or ISP route. In many small offices a simple router reboot, firmware update, or replacing an old access point can make a big difference.
3. Refresh the Outlook client
Ensure Outlook is up to date. Try temporarily disabling add‑ins (especially third‑party ones for encryption, CRM, or mail tracking) to see if performance returns. Rebuilding the Outlook cache (delete and recreate the OST) often fixes sluggish search and slow sync.
4. Reduce mailbox weight
Encourage staff to archive older messages into online archive or move large attachments to OneDrive and send links instead. Smaller, tidy mailboxes search and sync faster. A sensible retention policy helps keep inboxes manageable without losing important records.
5. Simplify rules and shared setups
Examine inbox rules and shared mailbox configurations. Consolidate or remove redundant rules and avoid multiple server‑side rules running at once. Shared mailboxes are handy, but poorly configured shared access can slow delivery.
6. Watch for background jobs
Automated scripts, bulk migrations, or sync tools can cause intermittent slowness. If you’ve recently migrated mailboxes or rolled out a new app that touches mail, pause it briefly to see if performance improves.
7. Use the right tool for mobile
On phones, use the official Outlook app rather than native mail apps in many cases. It’s better optimised for Exchange and handles sync more efficiently.
When to escalate to an expert
If you’ve tried the basics and users are still waiting, it’s time to escalate. Persistent slowness that affects multiple users during core business hours often involves more complex configuration or intermittent network issues that need professional diagnosis. Getting it sorted quickly reduces lost time and preserves client confidence — which matters for billing and reputation.
If you’d rather have someone else take the pain, consider a focused support engagement for Microsoft 365 rather than an open‑ended contract. For a straightforward route to reliable managed help, our experience shows that looking for targeted Microsoft 365 support for businesses can be the fastest way to restore productivity without a long wait.
Small changes that make a big difference
Some of the easiest fixes are low cost: replace a decade‑old router, push Outlook updates, run a short training session to stop staff sending heavy attachments, and introduce a basic retention policy. Those items rarely need big budgets, and they deliver visible results.
Real‑world caution
Be wary of blanket fixes like “just migrate everything again” unless you’re sure of the cause. Repeated migrations can create more problems than they solve if the root cause is local devices or network routes. I’ve seen firms in Leeds and Surrey chase migrations when a simple antivirus rule on a workstation was the culprit. (See our healthcare IT support guidance.)
FAQ
How quickly can email speed be restored?
It depends. A local Outlook client fix or router swap can be done in a morning. More complex issues involving mailbox size, rules or throttling may take a few days of diagnosis and staged fixes.
Will moving to Microsoft Teams reduce email problems?
Teams cuts down on internal email, but it doesn’t remove the need for reliable email. Teams is a complement: it reduces volume but doesn’t fix sync or delivery issues with Exchange.
Can we fix slowness without downtime?
Mostly yes. Many fixes — cache rebuilds, rule changes, archiving, router swaps — can be done with minimal disruption. Larger changes like migrations need planning, but the goal is to avoid interrupting billable work.
Is this a security issue?
Not usually. Slowness is typically performance related, but security software or content filters can slow mailflow. It’s sensible to include security checks in any diagnosis to rule out blocked or delayed delivery.
Should we keep PST archives?
PSTs are fragile and slow. If possible, move important archives into cloud archive or SharePoint/OneDrive, and avoid relying on local PSTs for day‑to‑day work.
Slow Microsoft 365 email is fixable, and the right combination of quick checks and practical changes gets most businesses back to normal without drama. If you want a focused fix that saves time and restores reliability, a short, outcome‑driven support engagement can be the calm, cost‑effective route to fewer interruptions and better credibility with clients.
