The error “can’t create user output file” usually means the mailbox or account has reached its storage quota and the mail system can no longer write new data for that user.

What a quota is

A quota is the storage limit assigned to a mailbox or user account. It controls how much data can be stored, including:

• emails
• attachments
• calendar items
• contacts
• notes
• deleted items still retained on the server

Quotas are used to keep the mail platform stable, responsive and manageable as well as reducing the risk of exposure due to compromise. A fuller explanation of why very large quotas are discouraged is available here:

• Limiting your Server Quota by Design

What the error means

When the system reports “can’t create user output file”, it generally means:

• the mailbox is over quota, or
• it is so close to the limit that the server cannot create the files needed to accept and store new mail

In practical terms, this means:

• new email may be rejected
• no more email can be received
• sending may also be affected in some cases, depending on the platform and client behaviour

How to resolve it

The issue is normally resolved by reducing mailbox usage or increasing the available storage.

Option A: Archive some email

Archiving moves older email out of the live mailbox so the server quota is reduced while important messages are still retained.

Typical approaches include:

• moving older messages into a local archive file in the mail client
• exporting historical mail to an archive mailbox or file
• storing older messages in a document management or retention system

Good candidates for archiving are:

• large folders such as Sent Items
• messages with large attachments
• old project mail no longer needed day to day
• automated reports and notifications

After archiving, allow time for the mailbox to re-synchronise and for space usage to update.

Option B: Delete some email

Deleting messages frees space more quickly, especially where large attachments are involved.

Focus on:

• Deleted Items
• Junk or Spam folders
• Sent Items
• emails with large attachments
• duplicate or obsolete messages

Important points:

• deleting mail from a folder may not free space immediately if it remains in Deleted Items
• some systems also keep deleted mail in a recoverable state for a retention period
• emptying Deleted Items is often necessary before meaningful space is released

If the mailbox is severely over quota, deleting a small number of very large emails may be enough to restore service quickly.

Option C: Request more storage

If the mailbox genuinely needs more working space, request a quota increase.

This is appropriate where:

• the current quota is no longer suitable for the role
• regulatory or operational requirements mean more mail must be retained online
• archiving or deletion would not reduce usage enough

However, increasing quota should be considered carefully, as very large mailboxes often create long-term performance and management issues. The reasons are explained here:

• Limiting your Server Quota by Design

Summary

The error “can’t create user output file” indicates that the user has exceeded their mailbox quota to the point that the system cannot store additional mail. As a result, new email cannot be received until space is freed or the quota is increased.

The standard remedies are:

• archive some email
• delete some email
• request more storage