What are the limits for local email storage with Postbox?

Danielx386 August 4, 2014
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Right now I have a very large inbox archived with Mailstore Home Edition. I would like to move from using Mailstore as it appears that there is a limit of the amount of emails that I can store. Yes it is for personal use as I have several email addresses over the years.

The other day I picked up a copy of Postbox and I’m looking at moving everything to that. My only question is – can the mbox format handle very large mailboxes? What the max size can mbox handle?

Thank-you :)

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  1. Oron J
    August 4, 2014 at 9:04 am

    Jeff's covered most bases. I would just like to add that although in principle there's no (practical) file size limit, at least in NTFS, performance-wise it's much better to keep the mbox files - and particularly the inbox - from getting too large.

    Very large mbox files require large indices. The indices take longer to update and longer to search (it's more a function of the number of messages than the size in MB, but the two tend to go hand in hand). If you use IMAP, then sync'ing will take much longer for a long list of messages than for a short one. Finally, compacting a large mbox file will clearly take longer than compacting a small one, and the same is true with copying them (as in backing them up).

    The solution I recommend to many of my clients is to keep the inbox really small (one or two screenfuls if possible), and using a separate folder (let's call it "archive") for messages that are no longer active, but which don't belong in an obvious folder. This is not only better from the technical angle, but will make it easier for you to find relevant messages in your inbox.

  2. Jeff F
    August 4, 2014 at 8:04 am


    The only constraint concerning the maximum emails you can download with Postbox is the amount of hard disk space available. Windows implements separate maximum files per directory limitations which is about 4 billion. If you have more than 4 billion files that you need to store in a single directory, then you may want to consider creating a few separate directories separated by year/month, etc.

    You're most welcome!

    Postbox - Download Messages for Offline Use (IMAP)

    • Danielx386
      August 4, 2014 at 9:01 am

      Thank-you for pointing that out, maybe I'm reading too much into it, does the mbox format have the same issue as the PST format if it get too large?

    • Jeff F
      August 4, 2014 at 9:40 am

      That's a separate question, Daniel ;)

    • Jan F
      August 4, 2014 at 1:24 pm

      mbox is probably the most fragile format for emails as literately everything is written into one big plain text file.

      PST certainly isn't as resilient as it should but since it's a container format it is easier for an application (well, Outlook) to handle once it grows over several GB.