I have a habit of never deleting an old mail (which proved useful many times over the years), I keep a busy digital life, and I am involved in many Web projects which involve receiving lots of email. As a result of all this, I have about 15 GB of emails dating back to 2007 on my computer, spread over 10 different email accounts. I am currently using Windows 7 and I will be getting a Windows 10 laptop next year.

At first I was using Thunderbird to manage the mail, but about three years ago it finally collapsed under the weight of my emails, simply locking up upon startup. I switched to its paid sibling Postbox, which is a bit better but as the mail size grows, it is too getting more and more laggy. Every few minutes it will hang for a dozen seconds and become totally unresponsive, which makes working with email extremely frustrating. Especially when I send an email, I am guaranteed a 10-15 seconds freeze before I can work with the program again.

Also, the text search capabilities of Postbox are very poor. I often search through my old emails to find a message, but I found out that Postbox is often unable to search through the email body, and sometimes even through email subjects or sender names. Also new emails are not searchable until Postbox indexes them, which it starts to do only when the computer is idle, but when I come back to the computer, again Postbox is hanging for a minute or so before it stops indexing and becomes responsive again. Finally, its search indexes get corrupted from time to time, which requires manually reindexing the offending mail folder, which with the size of my inboxes takes several hours during which the program again is unusable. For some reason reindexing is all-or-nothing, I cannot reindex a folder partially but I must wait until it goes through all messages.

All in all, I decided to look for an alternative email client. It would need to be able to handle such amount of email efficiently and provide adequate text search capabilities, plus of course it must be able to import my old mail from Postbox (which is in the same format as Thunderbird uses). As I said, I am a Windows user so the program must run under Windows. It does not need to be free, I understand that paying for such crucial software may be needed.

  • 2
    Have you tried creating folders and sub folders? Say by year first, maybe month,and then possibly by project. Either that or have a project folder and periodic archive folder. 1+gb mailboxes take a long time to index as you have found out. I would limit my self to no mailbox >500mb (maybe less) before creating a new one. A ssd instead of hard drive would dramatically increase your IO speed and reduce lag also. Most mail clients like thunderbird can automatically search across mail folders.
    – cybernard
    Dec 23, 2015 at 17:16
  • Also how much RAM do you have?
    – cybernard
    Dec 23, 2015 at 17:25
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    Yes, I partition my inboxes into yearly folders. I have a SSD disk and 6 GB RAM, of which Postbox takes about 1 GB (per Process Explorer). But when Postbox hangs, there is no disk access so the disk is not a bottleneck. I suspect the biggest problem is that Thunderbird (and thus Postbox) stores mail in "Unix mailbox" type files, which are extremely inefficient.
    – Michau
    Dec 24, 2015 at 22:24
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    The actual format name is Mailbox and the alternative is MailDir. You need more memory, at least 8gb now, 16gb within 3 years. The problem is Postbox is probably asking Windows for more memory, and Windows says nope I don't have that much to give you, and that doesn't show up one the usage counter. Then postbox stalls as it tries to negotiate memory from windows, and windows swaps out to the slooow swap file. Are any of your folders >500mb?
    – cybernard
    Dec 25, 2015 at 1:40

3 Answers 3


I have not used this personally, but I know Steve Gibson from GRC uses MailStore Home.

Quoting from episode 565 of Security Now:

It now has 2.5GB of my past email archived. It is itself a pop3 and/or IMAP client, so you can just aim it at your server, and it'll suck things in and index them. It can be a central archive for all your email. It can do Gmail, Yahoo Mail. It knows about all versions of Microsoft Outlook, Windows Mail, Windows Live Mail, Exchange Server, Office through 365, Mozilla Thunderbird, Sea Monkey, also PST, EML, and other files. It can suck those in, as well. And it runs perfectly next to my creaky old Eudora v7.1. Instantaneous index keyword search of all of my back email, which is really handy.

He has this set up to automatically import all his incoming mail, excluding attachments.


I don't know if you would be open to this approach, but try getting a new email account from FastMail. Their web, iOS and Android clients are extremely efficient at handling large mailboxes, in my experience. I believe they use a relatively new sync protocol called JMAP.

Looks like you would need the Enhanced plan (15GB of storage, $40.00 USD per year) or the Premier plan (60GB of storage, $120.00 USD per year).

Once you have your new account set up, use FastMail's IMAP Migrate tool (under Settings) to cleanly import all your old emails, with headers intact. (I don't know why this tool slipped my mind when I first posted.)

Note that FastMail lets you set up multiple personalities under the same account. So you could have 5 or more different email addresses that you send from, and an unlimited number of aliases that you receive mail to, all under the same plan.

  • FastMail a great webmail that I used in the past. The search through a mailbox is extremely fast. However, I see several issues using FastMail for my work. Having ~5 domain names and about 20 mail accounts, using Fastmail would cost a lot. FastMail increased its prices over time. The cheapest plan was free, then 10$/year, now 30$/year. There is no guarantee that FastMail won't go on increasing its prices. A third issue is confidentiality. Some years ago, FastMail had been purchased by Opera Software.What would happen if for instance FastMail was purchased by Alphabet (Google)?
    – OuzoPower
    Nov 23, 2017 at 13:12
  • @OuzoPower The people that run FastMail are extremely privacy- and security-conscious. The whole point of the service is that you pay for it so that you don't have to see ads or have your data sold to third parties. As such, I highly doubt that FastMail would ever be sold to Google or the like. Apr 22, 2018 at 3:14
  • 1
    JMAP could be the future.
    – guettli
    Nov 19, 2019 at 13:25

Facing similiar issues to you, I'm currently using The Bat! Professional (version from Ritlabs with a little above 15 GB of emails and ~20 mail accounts. Configuration is Windows 7, SSD, 4GB RAM, and it is fluent.

There are things that I like and others that I dislike.

Using sub-folders helps sorting emails, but requires manually sorting emails.

Searching amongst emails is unfortunately very slow with The Bat! as the content of emails is not indexed and the interface is poor in this regard.
The Bat! stores emails into its own archive format. The search tool does not allow cross-accounts search, nor setting the date range.

I would suggest looking at Claws Mail or Sylpheed as they are told being fast. For a selfed-hosted webmail. I believe, they store emails into individual files, which may possibly help using third-party tools to index and/or search through emails.

Another possible candidate could be Zimbra as it does have cross-accounts search. From screenshots and online demo, both the desktop client and the webmail seem polished. I also read positive opinions about it.

  • Note: Zimbra belongs to Synacor. On slide 11 of Synacor's Investor Presentation (investor.synacor.com/investor-relations/…), Synacor tells about an "Open source program to monetize 400 million existing mailboxes". A priori the monetization seem being done through Support and additional tools like backup (and not via advertisement), despite Synacor bein also active in the adversing market.
    – OuzoPower
    Nov 23, 2017 at 16:29
  • Warning for Zimbra: read the comment from Enrico Weigelt here: open-source-guide.com/Solutions/Infrastructure/…
    – OuzoPower
    Nov 23, 2017 at 16:42

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