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.