How to Compress MBOX File Size on Windows and Mac

Why Compress a Large MBOX File?

Large archives cause four problems and a smaller file fixes them.

• Slow Mail Client. Mail clients open and search a large mailbox slowly.
• Storage Limits. FAT32 storage devices store no file above 4 GB and cloud plans cap your space.
• Slow Transfers. Big files take long to copy or upload.
• Backup Size. Every backup copy of a large file uses the same space again.

How Does an MBOX File Store Attachments?

MBOX saves a mailbox folder as a single text file. Every email holds its attachments as encoded text. One email with a large PDF adds the size of that PDF to the file.

Learn more in our MBOX file format guide.

What to Check Before You Compress MBOX Files?

Four checks protect your emails before you start.

• Back up the MBOX file and work on the copy.
• Save attachments you still need because removal deletes them from the file.
• Close Thunderbird or Apple Mail before you copy the file.
• Free up disk space equal to the file size because the new file sits next to the original.

How to Reduce MBOX File Size Manually?

Three free methods compress MBOX file data. Two delete attachments inside a mail client and one packs the file into an archive.

Thunderbird (Free, Deletes Attachments in the Client)

Thunderbird saves every folder as one MBOX file. Deleting attachments and compacting the folder shrinks that file.

1. Open Thunderbird and select the folder that matches your MBOX file. Add the file under Local Folders first with the Thunderbird steps in our Split MBOX File guide.
2. Open an email that carries a large attachment.
3. Right click the attachment name and choose Delete. Choose Detach to save a copy on your PC before Thunderbird removes it.
4. Select Attachment in the Quick Filter bar to list only emails with attachments. Repeat the step for the next emails.
5. Right click the folder and choose Compact. Thunderbird rewrites the file and frees the space.
6. Open the Local directory path from Settings and check the new file size.

Handling one email at a time takes long. Use this method for small mailboxes.

ZIP and Gzip Archive (Free, Saves Storage Only)

Archives shrink the file on your PC but mail clients cannot open an archive. Extract the file before you import it.

1. Close your email client and locate the MBOX file.
2. Right click the file. Windows 11 users choose Compress to ZIP file and Windows 10 users choose Send to > Compressed (zipped) folder. Mac users choose Compress.
3. Linux and Mac users can use the command below in Terminal. Option k keeps your original file and creates big.mbox.gz beside it.

gzip -k big.mbox

4. Replace big.mbox with your file name and compare the sizes of both files.

Images and PDFs carry their own compression so archives shrink them less than plain email text.

Apple Mail (Free, for Mac)

Apple Mail removes attachments from selected emails and keeps the email text.

1. Open Apple Mail and select the mailbox with large emails.
2. Press Command + A to select every email in the mailbox.
3. Click Message in the menu bar and choose Remove Attachments.
4. Select the mailbox, click Mailbox and choose Export Mailbox. Select a folder to save the new MBOX file.
5. Check the size of the exported file.

Need the full export steps? Follow our Import and Export MBOX in Apple Mail guide.

How to Compress MBOX File With MBOX Compress?

EmailDoctor MBOX Compress removes attachments from MBOX files and saves a smaller MBOX file in a folder you select. MBOX, MBX and MBS files open in the software and MBOX files with no extension work too. Filters narrow the attachments by date, size and type.

Windows 11, 10, 8 and 7 work with the software and so do Windows Server editions. Install .NET Framework 4.5 or above before you start.

Six features help with large files.

• Many File Types. Compress MBOX, MBX and MBS files and MBOX files with no extension.
• Two Removal Modes. Remove attachments or remove them and save copies in a folder.
• Date Filter. Include or exclude a date range.
• Size and Type Filters. Set Upto or More Than from 1 KB to 100 MB and include or exclude file extensions.
• Search Files. Find MBOX files on your disks when you forget the location.
• CSV Report. Get a report for every compressed file.

Compress an MBOX File on Windows

1. Start MBOX Compress. Windows users right click the shortcut and choose Run as Administrator. Main screen tabs are Add, Select, Filter and Compress.
2. Select MBOX/MBS or MBX to match your file type.
3. Click Add File(s) for one file, Add Folder for a full folder or Search Files to find MBOX files on your disks.
4. Check the file path and the size in KB under File Path. Select a wrong file and click Remove or click Remove All to clear the list.
5. Select Remove Attachments under Options to delete the attachments from the compressed file.
6. Select Remove and Save Attachments in a Folder to keep copies of the attachments. Click Next.
7. Review the folder tree on the Select tab. Untick the folders you want to exclude.
8. Click Change under Destination Path, select the output folder and click OK. Click Next.
9. Open the Filter tab. Select Include under Date Filter to remove attachments only from emails in a date range. Click Add, enter the Start Date and End Date and click Add again.
10. Select Exclude to skip a date range and keep its attachments.
11. Tick File Size under File Filter. Select UpTo or More Than, enter a number and select MB or KB.
12. Tick File Type and select Include or Exclude. Type extensions separated by commas like txt,doc,pdf.
13. Click Next to start the compression.
14. Watch Success Count, Fail Count, Attachment Count and Status on the Compress tab. Click Stop to end the process early.
15. Click OK on the message that says the MBOX files have been compressed successfully.
16. Go to your destination folder and find the new MBOX file next to a Report folder. Compare the new file size with the original.

    Try the free demo on one small MBOX file first. Demo mode compresses 15 items per folder of every file.

    MBOX Compress works on Windows only and Mac users follow the Apple Mail method above. Need to extract attachments instead of removing them? Follow our Extract Attachments From MBOX File guide.

Which Method Suits Your MBOX File?

All four methods compress MBOX file data but they differ in what they remove and where they work.

I suggest Thunderbird or Apple Mail for a small mailbox, an archive for storage and MBOX Compress when you need many files or filters.

What Problems Appear While Compressing MBOX Files?

Six problems stop MBOX compression.

• File Size Does Not Shrink. Right click the folder in Thunderbird and choose Compact because deleted attachments free space only after compaction.
• Archive Will Not Open. Extract the ZIP or Gzip file first. Mail clients open MBOX files only.
• Attachment Needed Later. Restore the file from your backup. Choose Detach or the Remove and Save Attachments in a Folder option next time.
• Folder Missing in Thunderbird. Close Thunderbird, remove the file extension and copy the file to the Local directory.
• Software Does Not Start. Right click MBOX Compress and choose Run as Administrator. Confirm that .NET Framework 4.5 or above exists on your PC.
• Demo Compresses 15 Items. Free demo mode compresses 15 items per folder. Buy a license to compress the full file.

Frequently Asked Questions

Final Words

People compress MBOX file archives because attachments fill the space. Free methods delete attachments in a mail client or pack the file into an archive.

Back up the file and test one small file first. Open the new file in Thunderbird and check the email count and attachments before you compress MBOX file copies in bulk.

MBOX Compress then handles large archives. Filters select the attachments and the report shows the result for every file you compress.