Just because you have a large mailbox doesn’t mean you should keep everything forever. When your mailbox gets too full—especially over 25GB—it can slow down Microsoft Outlook, make searches take longer, and even cause email sync issues with your phone or laptop. It’s like trying to find one document in a room full of paper—it takes longer and gets frustrating fast!
Keeping your mailbox tidy also helps behind the scenes. A messy mailbox makes backups and updates slower, and storing old, unnecessary emails can be risky—especially if they contain sensitive info.
To help you manage your mailbox effectively, we recommend the following monthly maintenance steps:
| Task | Time to Complete | Benefit |
| Delete emails older than 12–18 months (especially with attachments) | ~15 mins | Reduces clutter and improves Outlook speed |
| Empty your Trash and Junk folders | ~2 mins | Frees up storage instantly |
| Review folders with lots of emails (like Inbox or Sent) | ~5 mins | Helps Outlook search and load faster |
| Avoid using your inbox as a file cabinet | Ongoing | Makes email easier to manage and search |
Here are a few quick things you can do to keep everything running smoothly:
Use Folders to Stay Organized
- Create folders by project, client, or topic so you don’t keep everything in your Inbox.
- This makes it easier to find things and reduce the size of large folders like Inbox or Sent.
Sort by Size
- In Outlook, sort your emails by size to quickly spot the biggest ones.
- Tip: Look under “Search Tools” > “Has Attachments” + sort by “Size.”
Save Attachments Outside of Email
- Open emails with large attachments and save the files to your desktop, OneDrive, or shared drives.
- Once saved, delete the email or remove the attachment (right-click and choose “Remove Attachment”).
Limit Use of “Reply All”
- Long threads with lots of attachments get duplicated every time you hit “Reply All.”
- Respond only to those who need to see it, and avoid sending attachments back and forth.
Empty Sent Items Regularly
- Sent Items can fill up fast—especially if you send files.
- Delete old messages you’ve sent that no longer need to be saved.
Avoid Using Email as File Storage
- Don’t use email to store things like photos, forms, or scanned documents long-term.
- Files should live in a shared drive or a proper document system—not your mailbox.
Set a Weekly 10-Minute Cleanup Reminder
- Block 10 minutes on your calendar each week for “Mailbox Cleanup.”
- It’s easier to stay on top of it than doing a big purge once or twice a year.
Watch for Mailbox Warnings
- If you get a warning about mailbox size, act immediately.
- Waiting too long could affect your ability to send or receive email.
Don’t Store Personal or Sensitive Info in Email
- Remove things like personal documents, old passwords, or sensitive files.
- Keeping them in your mailbox increases risk if your account is ever compromised.
The best mailbox size for Microsoft Outlook is under 10GB for optimal performance, with a strong recommendation to keep it below 5GB.
Why?
- Under 5GB: Fastest performance, quick search, minimal sync issues.
- 5–10GB: Generally stable, but performance may slow depending on folder size and system resources.
- 10–25GB: Outlook and Zimbra performance begin to degrade—slower searches, freezes, sync delays.
- 25GB+: High risk of crashes, corrupt profiles, failed indexing, and major sync problems.