Recommendations for software (Windows platform) for local e-mail backup in non-proprietary format?

A requirement that the German finance ministry came up with last year was that for businesses (including my small part time web business) not only the accounting information/invoices etc. were to be backupped in an unalterable form (that’s what my tax accountants have always been doing), but also other relevant business records.
For purposes of a possible audit, which for my business size is to be expected about once every other century, but still I am law abiding and having backups is a Good Thing anyway.

Playing it safe, I have since downloaded my e-mail accounts via IMAP with Windows Live Mail (2011 version), made a monthly .7z archive of the directory and burned it, together with other business data, on a BluRay disc every month. The BluRay disc is almost filled to capacity these last few months.

My concern is that Windows Live Mail is outdated by now and seems to store mails in a proprietary format (plain text which is good, but some parts of the folder structure/metadata are opaque to me, and I’d probably have a hard time restoring lost e-mails from the backup.)

What I am looking for is some software that

  • runs on Windows 7 and Windows 10
  • can download e-mail via IMAP, over secure connection, without mailbox size/e-mail number restrictions
    - stores the mailboxes in an open format, that other e-mail clients also use (i.e. I could migrate to some other software while keeping the data, and can have some confidence that in some years software to read the format will still be around)
  • has an useable search function for its offline data
  • is affordable or free.

Any suggestions for that purpose?

It sounds like you’re really just looking for a mail client - they can be configured to download a copy of mail and store it, index it for searching, etc.

So you could just install Thunderbird somewhere and configure it to receive email, but not send.

For cross-platform availability of the messages (i.e. if you want to take them somewhere else), there are plugins such as ImportExportTools

I’m not sure about other clients being able to use it (though I bet they can), but Mozilla Thunderbird is an email client that you can backup (the backup program MozTools?) will export all your emails and settings. Since it’s Mozilla it’s free and since it’s a client it runs on your local machine (offline).

It wouldn’t surprise me if you could import all this back to another client but even if you can’t, anyone can download Thunderbird for free and import/search the data.

Can’t you just use Microsoft Outlook?

Mainly because of the propriatary storage format. Also I’d have to buy a license (not having Office on my PC).

A few more things, I’d still go with Thunderbird (and the program to back it up is MozBackup, not MozTools).
I’m not sure what you mean by unalterable. I read that to mean the data can’t be changed, but you then said you are adding on to the disc each month. Is it just that you can’t go back and change data already written? (It’s been so long since I’ve burned any type of CD/DVD I don’t remember how all the different types work).
Either way, I’d suggest having a few of them and alternating. For example, if you have three, you could back up on the first one in January, the second one in Feb and the third one in March then start back over in April. This way if you need to go back and your drive is corrupt, instead of losing everything, you’ve only lost the last month and you can go to the month before.

I just keep a handful of cheap jump drives and every few days do a backup and toss them in my car. Once they’re all in my car I bring all but one in and start over. With 4 or 5 drives and 3 backups per drive I’ve got quite a bit saved.
(ETA that last part is for Quickbooks, I backup Thunderbird and/or my entire drive less often. It takes a while and QB is more important).

Unalterable in the sense that I write the data (compressed into archives) on a BluRay disk and cannot change it later. At present everything fits onto a single disk so that’s 120 disks that I need to store for a 10-year retention period.

I use Mailstore Home, but I believe that is only for home, but not business users. They also have commercial products for business:

Are the discs sealed at all after writing? (what would prevent you altering the data and writing a new version of an old disc, then destroying the original)?

You are right, I’d be able to fake past data that way if I went to a lot of trouble and work (but as my business is one with four figure profits in good years there is a limit to the level of precautions against myself I am going to have). I supose businesses under a lot of scrutiny such as banks would need to lock up their backups in a way where the person making the backups cannot get them back out of storage.