When using Microsoft Outlook, very large mailboxes can cause performance issues and even file corruption. This is not specific to our hosting platform — it is a limitation of Outlook itself. Even if you move your email account between shared hosting, dedicated servers, or cloud services, the problem will persist if the mailbox is excessively large.
Understanding Outlook data files
Outlook stores a local copy of your mailbox in a data file called an OST (or PST). Modern versions of Outlook set a default limit of 50 GB per file, and although this can be increased through advanced settings, stability drops significantly once a mailbox grows beyond 20–30 GB. At 70 GB or more, users frequently experience:
- Slow performance and freezing in Outlook
- Sync errors or missing mail
- Corrupted OST/PST files requiring a rebuild
- Very long times to repair or re-download the mailbox
Why this is a client software issue
The problem is not caused by your email host. Regardless of whether your mailbox is hosted on a shared server, a dedicated environment, or a cloud service like Office 365, Outlook still has to manage the data locally. If the mailbox is too large, Outlook will eventually struggle to keep it stable.
Options to reduce problems
- Limit cached data: Configure Outlook to only sync the last 12–24 months of email, instead of the full mailbox.
- Use archives: Move older mail into an Archive mailbox or a separate archive folder on the server.
- Switch to online mode: Outlook can be set to connect directly without creating a local OST file, although this depends on your internet connection quality.
- Regular archiving: Export older messages into local archive files so your active mailbox remains lean.
Alternative email clients
Other mail clients such as Thunderbird handle large mailboxes differently. For example, Thunderbird with Maildir storage keeps each message as a separate file, which reduces the risk of a single large file becoming corrupted. While no client performs perfectly with extremely large mailboxes, Thunderbird generally copes better with sizes above 50 GB.
Best practice recommendations
- Keep your active Outlook mailbox under 20–30 GB for best performance.
- Use archiving to manage older messages instead of keeping everything in the Inbox or Sent folders.
- Consider webmail or alternative clients for long-term archive access.
Managing mailbox size is an essential step for maintaining a reliable and fast email experience. If your mailbox is currently over 50 GB, we strongly recommend archiving and trimming it down to avoid future corruption.
