Clear the biggest items first: large attachments, Deleted Items, and Sent Items. Microsoft 365 business plans include an online archive, which can move older mail out of the main mailbox automatically without deleting it.
Common causes
Large attachments
A few big attachments can take more space than thousands of messages.
Deleted Items never emptied
Deleted messages still count against your mailbox until emptied.
Years of mail in one mailbox
Long-time users can reach the mailbox limit without an archive.
Shared mailboxes and calendars
Shared mailboxes have their own size limits.
How to fix it, step by step
- 1
Empty Deleted Items and Junk
Right-click each folder and empty it.
- 2
Find large messages
Search for messages larger than 5 MB and save important attachments to OneDrive before deleting them.
- 3
Clean up Sent Items
Sent Items often holds many large attachments.
- 4
Turn on the online archive
Your administrator can enable an archive mailbox and a policy that moves older mail automatically.
- 5
Check the mailbox size
In classic Outlook, check File > Info to confirm space is freed.
When to call a technician
We set up archiving and retention so mailboxes don't fill up again, and make sure nothing important is lost in the process.
On-site support across Orange County · see our IT support, managed IT, Microsoft 365 support and cybersecurity services.
Frequently asked questions
How big is a Microsoft 365 mailbox?
Most Microsoft 365 business plans include a 50 GB mailbox, with an online archive available depending on the plan.
Does archiving delete my email?
No. Archived email stays searchable in Outlook; it's just stored in a separate archive mailbox.
What happens when my mailbox is full?
You can't send email, and eventually new incoming messages bounce back to the sender.