Why Deleting Emails Won’t Fix Your Full Mailbox – And What to Do Instead

If you work in industries like construction or design, chances are you send and receive a lot of emails—often with large attachments. This can quickly fill up your mailbox, especially for Microsoft 365 users, where the default mailbox size is 50 GB.

Most people’s first reaction is to start deleting emails to free up space. However, this is usually not effective and can even cause more issues. Here’s why:

Why Deleting Emails Doesn’t Always Work

At Excelero IT, we apply a default retention policy and enable legal hold for all client mailboxes. This ensures that all emails—even ones that are deleted—are still available in case they’re needed for future reference or legal purposes.

Here’s how it works:

• When you delete an email, it first goes to the Deleted Items folder.
• If you delete it from there, it moves to the Retention folder.
• Even if removed again, it’s preserved in a hidden Preservation Hold folder, accessible by system administrators.

All of these folders still count toward your 50 GB mailbox limit.

The Better Solution: Enable Online Archive

To manage mailbox size effectively, we enable Online Archive by default. This is a secondary mailbox with an additional 50 GB of storage. Emails older than two years are automatically moved into this archive.

Better still, users with Microsoft 365 Business Premium licenses get an auto-expanding archive, which grows beyond 50 GB as needed. The online archive mirrors the folder structure of your main mailbox, so it’s easy to find older emails in the same location—just in the archive instead.

Our recommendation: Let the automatic archive do its job. Don’t waste time manually deleting emails—it’s unnecessary and time-consuming.

A Common Trap: Shared Mailboxes and Local Limits

Another issue arises with shared mailboxes. Even if each individual shared mailbox is under the 50 GB limit, Outlook caches them locally on your computer. For example, if you’re connected to three shared mailboxes, each using 20 GB, your local Outlook file could hit 60 GB—exceeding the local file size limit.

This can prevent you from sending or receiving emails.

To fix this:

• Reduce the Outlook cache duration (e.g., from 12 months to 6, 3, or 1 month).
• Close Outlook, delete the local mail file, and restart Outlook to resync a smaller mailbox.

Looking Ahead: Try the New Outlook

Due to growing issues with mailbox sizes, sync errors, and file limits, Microsoft is encouraging users to switch to the New Outlook. This version operates entirely in the cloud—similar to the Outlook web app—and avoids many of the technical problems associated with the classic Outlook desktop version.

Check out our May edition of Tech Talk for a full review of the New Outlook.

Key Takeaways:

1. Don’t waste time deleting emails—automatic archiving handles this.
2. Search your Online Archive for emails older than two years.
3. Try the New Outlook to avoid sync and storage issues.
4. If you can’t send or receive emails, check local file size limits and shared mailbox cache settings.