How to Keep Your Mailbox Healthy and within the Limits
Smart Email Habits for every outlook user
Email is one of the tools we rely on every day but without a bit of housekeeping, your mailbox can quickly grow out of control. Large mailboxes make Outlook unstable and slow, can make recovery more difficult, cause mailbox sync issues and will eventually hit Microsoft 365 storage and Outlook file limits.
The good news is that keeping your mailbox tidy doesn’t require hours of admin. A few simple habits and some setup changes can make a huge difference.
Below are some best practices everyone can follow to keep Outlook email stable, organised, and stress‑free.
1️⃣ Delete the “big hitters” first - large attachments
Attachments are the number one cause of mailbox bloat.
Quick wins:
Sort your mailbox by Size and delete the biggest items first
Save important attachments to OneDrive or SharePoint, then delete the email
Empty your Deleted Items afterwards so the space is freed up
Why it matters: A single 20MB attachment can take up more space than hundreds of normal emails.
2️⃣ Use OneDrive links instead of sending attachments
Instead of sending a 10MB file to five people (which creates five copies), send a OneDrive link.
Benefits:
Uses almost no mailbox space
Always up to date — no more “v5_final_FINAL.pdf”
More secure: you can revoke access at any time
This is one of the easiest ways to reduce mailbox growth long‑term.
3️⃣ Clear out your Sent Items
Most people forget this folder exists, but every attachment you send is stored here too.
Best practice:
Sort by Size
Delete old sent emails with large attachments
If you need to keep the attachment, save it to OneDrive or SharePoint first
Remember to empty your Outlook Deleted Items when you’re done
4️⃣ Archive older email instead of keeping everything in your Inbox
Your Inbox should be for things you’re actively working on and not a storage vault.
Try this:
Create an Archive folder or ask IT to activate your online archive if your 365 license provides this feature
Archive anything older than say 6 months
Use Outlook’s Archive button for one‑click filing or leave it up to the auto archive feature for worry free ongoing filing
This keeps your Inbox clean and manageable.
5️⃣ Empty Deleted Items and Junk regularly
Deleting an email doesn’t free up space until it leaves Deleted Items.
Make it a habit:
Right‑click Deleted Items → Empty Folder
Clear Junk Email weekly
It takes seconds and can free up hundreds of MB.
6️⃣ Unsubscribe from newsletters you never read
Marketing emails can pile up quickly.
If you don’t read it, unsubscribe. If you might read it, create a rule to move it to a folder automatically.
7️⃣ Remove other users shared mailboxes
Having other users mailboxes attached to Outlook doesn’t increase the size of your Microsoft 365 mailbox online, but it can significantly increase the size of your local Outlook file (OST). When this files reaches its 50GB limit Outlook stops processing emails and becomes unstable.
Remove non-essential shared mailboxes from your Outlook
Ask IT to disable automapping if you don’t need the mailbox visible all the time in Outlook on your PC and add it manually when required
Or
With auto mapping disabled, just access the other persons shared mailbox via Outlook online
A smaller OST file = A more stable Outlook.
8️⃣ Don’t use your mailbox as long‑term file storage
If you need to keep a document attached to an email:
Save it to OneDrive, SharePoint or your CRM
Delete the email the document was attached to
Share the document from OneDrive if needed
Email/Outlook is not designed to be a document management system — OneDrive/SharePoint are.
9️⃣ Use Conversation View to delete entire threads
If a lengthy email chain is no longer needed:
Turn on View → Show as Conversations
Delete the whole thread in one go
This removes dozens of messages at once.
🔟 Know the limits
Microsoft 365 mailboxes have generous storage, but not unlimited and the default is 50GB. This can be increased through additional licensing and tweaks to the PC’s Windows registry to allow the Outlook (.OST) file to grow beyond its designed limits, but this invites issues and is not a long-term solution.
Keeping your mailbox tidy helps:
Prevent sync issues
Keep Outlook running smoothly
Avoids hitting 365 storage and local Outlook file size limits
Improve search performance and recovery time
A few minutes of housekeeping each month goes a long way.
We all use Outlook outside of its designed functions, keeping files in emails, creating large eleborate folder structures even within the deleted items folder itself! and send large emails that bloat yours and other users mailboxes. Breaking these habits by using some or all of the recommendations shown can help keep things running smoothly for everyone.
If you need help with email, achiving or email backups, get in touch with us today.
020 8770 0007 | info@s50.co.uk