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I have thunderbird 12.0.1 portable on windows xp. How can I compress email in thunderbird? I checked its storage area and found that emails are stored in plain text without any compressing.

There is about 10 gb of emails and that would be very helpful to shrink disk usage.

Clarification: Thunderbird Compacting is not compressing

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The easiest solution might be filesystem compression (e.g. NTFS). Just tell the OS to compress all files in the mailbox folder. – MSalters May 28 '12 at 22:13
I would advise against NTFS compression as it introduces massive fragmentation, and on newer drives with 4KB clusters, can dramatically increase space used. – user3463 May 28 '12 at 22:51
@Randolph: Isn't the fragmentation effect only true when frequently writing to large files? And since Windows Vista and 7 defrag automatically regularly, it shouldn't be that big an issue unless you're using compression on a VHD. Also, how would NTFS compression increase space used on 4KB cluster drives? The filesystem is supposed to write the compression unit to disk uncompressed unless compression reduces its size by at least one cluster. Having 4KB clusters should increase efficiency, though at the cost of random access speed. – Lèse majesté May 29 '12 at 9:29
If the individual file sizes are small, it will increase space used. I'm not familiar with Thunderbird's storage mechanism though. As for fragmentation, on NTFS-compressed files it is astronomical. Try it and see. Windows automated defrag only runs on Wednesday mornings at 1am by default, so if it's a laptop, and the machine is off, defrag isn't going to help. – user3463 May 29 '12 at 20:59
@user3463 compression does not increase space used, and small files are not compressed at all, no matter the attribute value. Also, 4kb clusters are default for NTFS from at least NT4.0. And Thundebird stores mail in 1 large file-per-folder + small thunderbird index + small windows search indexes – LogicDaemon Jun 4 '14 at 14:01

The option named "Compact folders when it will save over" under

"Edit->preferences->Advanced->Network & Disk space"

would allow you to enter a size, when the folder exceeds that size, the mails will be compacted.

Disclaimer: I gave the location of the option from my Linux machine, Probably it will be in some other menu for Windows' thunderbird.

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Compacting is not compressing unfortunately. It just physically removed emails marked as deleted. – misha nesterenko May 29 '12 at 10:43
1. Compacting is not compressing, as @mishanesterenko pointed 2. Size specified there is not size of a folder, but instead size of deleted emails in all folders. When it reached, compacting will remove them to free used space. – LogicDaemon Jun 4 '14 at 13:53

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