If you work with the Microsoft Outlook for Mac application, all your local mailbox details such as emails, attachments, calendar appointments, contacts, tasks, and notes will be stored in one unique database file format called OLM (Outlook for Mac).
Many Mac users find themselves facing the issue of a very big size of their OLM file reaching tens or even hundreds of gigabytes. An oversized OLM file not only takes up the internal SSD memory of your computer, but also may result in performance issues, slowing down, failures in backups and exports of the information.
The following information provides a detailed explanation of why your OLM file becomes so big and what are the dangers of having such a bulky email archive as well as how to decrease its size.
Why Does an OLM File Become So Large?
Knowing the reasons behind the increase in the size of the OLM database can help you better manage storage space.
- High Volume of Uncompressed Attachments: The contemporary email system is based on rich media, high resolution pictures, PDF files, spreadsheet, and presentations. While web-based email servers store compressed versions of attachments, Outlook for Mac stores uncompressed copies of each attachment in the local OLM database.
- Deleted or Junk E-mails: Pressing the Delete button on your email doesn’t mean that it will be immediately deleted from the local database. The emails will be stored in the Deleted or Junk E-mail folders occupying space within the OLM database until they get permanently deleted.
- Multiple Copies of Email Messages and Long Email Threads: In long email threads, there are often repeated images, signatures and previous messages. Having been storing your emails for many years, you will find numerous duplicates of message parts within the file structure.
- Management of Multiple Accounts: In case of using more than one email account such as Exchange for work and Gmail, as well as Office 365, through a single Outlook for Mac application, the creation of an OLM file leads to amalgamation of all such accounts into one huge file.
- Fragmentation of the Database: After several months or even years of consistent usage, during which emails have been constantly added and deleted, the Outlook database will become fragmented.
Risks of an Oversized OLM File
Using your Mac with a significantly oversized OLM file poses various risks:
- Trying to export an OLM file of size larger than 30-50 GB using the native Mac Outlook software often results in the application freezing or hanging up during the process.
- Large archives will fill your SSD drive quite fast, which will lead to the entire Mac slowing down and inability to make Time Machine backups.
- Big single database files have a statistically higher chance of getting corrupted due to sector errors in case of sudden loss of power supply or system crash.
Practical Ways to Reduce OLM File Size
To shrink your OLM archive and regain control over your Mac’s storage, follow these step-by-step reduction strategies:
Method 1: Empty Trash and Junk Folders Permanently
Before generating an OLM export, clean out unnecessary folders:
- Launch Microsoft Outlook for Mac.
- Right-click the Deleted Items (or Trash) folder in the sidebar and select Empty Folder.
- Right-click the Junk Email folder and select Empty Folder.
- Navigate to your Drafts folder and clear out outdated or abandoned email drafts.
Method 2: Detach and Save Large Email Attachments Externally
Since attachments account for the majority of OLM file weight, saving attachments locally and removing them from messages will dramatically reduce the file size:
- In Outlook for Mac, use the search bar to filter emails by size (e.g., search size:>10MB).
- Open messages containing heavy attachments (PDFs, ZIPs, videos).
- Right-click the attached file and select Save All… to store it on an external hard drive or cloud storage (e.g., iCloud, Google Drive).
- Remove or delete the heavy message (or the attachment) from the email thread.
Method 3: Export Selective Folders Instead of a Single Master Archive
Exporting your entire mailbox into one monolithic file increases the risk of file size bloat. Instead, create separate, smaller OLM files categorized by year or project:
- In Outlook for Mac, go to Tools > Export.
- Uncheck items you do not need (such as Tasks or Notes if you only need emails).
- Filter or select specific folder sub-trees (e.g., export only Archive 2023 or Project Alpha).
- Save separate, smaller .olm files.
Method 4: Compact the Outlook Database
Rebuilding and compacting your Outlook profile clears out fragmented database blocks:
- Close Microsoft Outlook.
- Open Finder, navigate to Applications, locate Microsoft Outlook, right-click it, and select Show Package Contents.
- Navigate to Contents > SharedSupport and double-click Outlook Search Repair (or use the Microsoft Database Utility on older Office for Mac builds).
- Re-index and repair your profile to remove orphaned database sectors before exporting your OLM file.
Solution Tip: Use MacMister OLM Converter Tool to Manage & Split Large Archives
In case your OLM file is too big to be opened by Outlook for Mac or native exporting is freezing because of archive bloating, using the special tool MacMister’s Mac OLM Converter Tool will solve this problem easily.
How MacMister OLM Converter Deals with Archive Bloating:
- Rather than loading huge and unmanageable database, MacMister OLM Converter shows all internal folder structure of your OLM archive, so you may convert emails from some particular date or from certain folders (for example, only Inbox or project-related archives), skipping unnecessary folders.
- With the help of MacMister OLM Converter, you will be able to split huge, closed OLM archive into popular formats. It allows to convert Mac OLM to PST, EML, MBOX or PDF. It helps save some space on disk and gives an opportunity to view single emails in Apple Mail or Thunderbird without having bulk OLM database file.
- In case of orphaned, damaged or too big OLM file that is not manageable by Outlook for Mac, MacMister OLM Converter analyzes file sectors independently, maintaining attachments, folder structure and metadata of your email messages without freezing or crashing.
Conclusion
An overgrown OLM file is usually caused by multiple attachments, a lot of garbage, redundant emails, and broken database blocks. With regular deletion of trash, storing large attachments offline, and selective export of folders, you can maintain your Outlook for Mac database in a compact and efficient state.
In case of an old, large, or poorly managed OLM archive, using the MacMister OLM Converter Tool will enable you to filter out, convert, and export your important data without putting strain on your Mac memory.
