Mail storage structure and rules

MrHoffman wrote:

And as is their wont, Apple has changed the structure and format of the mail database on various occasions, and without notice.

That's not really an Apple thing. Anything inside a "Library" folder is always an implementation detail. End users should never, ever go into a Library folder.

So why not just export them and let it do all of that for you? You'll get back the message as it was originally delivered.

etresoft wrote:

All of that is internal information about Apple Mail. Nobody outside of Apple can answer those questions. Nobody inside Apple ever would. Why do you want to know?

And as is their wont, Apple has changed the structure and format of the mail database on various occasions, and without notice.

TCambazard wrote:

I'm trying to consolidate data from an mac to a eml format, and yes I know there are tools out there but I would need some understanding of the process.

Poking around in the Library folder is not the way to do that. Instead of consolidating anything, all you will do is corrupt your Mail database.

I don't even know what you mean by "consolidate". Normally the way it works is you keep your mail in an online e-mail service like iCloud, Microsoft Exchange, or Gmail. Then you just use your Mail. If you are trying to do something else, then it would be best to start by describing exactly what you are trying to do. If all you want to do is corrupt your e-mail database, then keep going. If you want, I can recommend some 3rd party software that will corrupt your data much faster than you could ever do manually.

TCambazard wrote:

I'm trying to consolidate data from an mac to a eml format, and yes I know there are tools out there but I would need some understanding of the process.

“Corrupting your mail database” would be the shorter description of that goal.

EML format is a Microsoft-developed message format, and used for migrating messages. macOS Mail app supports exporting to that message format, but does not use that format internally.

EML is ill-suited for directly accessing messages, as the EML files need to be parsed and their contents extracted to separate files before, for instance, the images and other file attachments can then be accessed. Or in-memory parsers need be written for ~everything, of course.

If you want to migrate to some other mail client such as Microsoft Outlook (and with Outlook, then probably also from macOS to Microsoft Windows), then select the appropriate tools and migrate to that. EML handles message migrations well.

TCambazard wrote:

I'm trying to consolidate data from an mac to a eml format, and yes I know there are tools out there but I would need some understanding of the process.

To clarify here, I'm sincerely asking you to describe what you are trying to do. Please be specific. "Consolidate" is not specific. What are you trying to do?

I can tell you what I do. I only keep the past couple of years of e-mail messages on the servers. Every couple of years, I export my old message to "On my Mac" mailboxes, organized by year. Then, when I want to setup a new computer, I have to remember to manually export all of those "On my Mac" mailboxes as "mbox" files and then re-import them on the new computer. When I setup a new computer, I do it with iCloud. Migration or Time Machine is only appropriate when you are moving content and the old system will be put out of service or completely reconfigured. I tend to keep my old computers around for testing or backwards compatibility. If I copy my data from those old computers, it will subtly corrupt lots of low-level things. So I let iCloud do what it does best and manually copy the rest. I do this maybe once every 3 years or so, so it's not a burden.

The internal format of the Apple Mail database or individual files may be technically interesting, but not really important. You can export individual messages to elmx files and use the Finder as a database for organization. This way, they will act as document files that can be opened with mail. But their internal structure is an implementation detail that is private to Apple. The same is true of any document format from Apple or any other developer.

TCambazard wrote:

Export from where? I have an iOS (an iMac)'s content, with the tree structure but I am not on the iMac itself with an access to the Mail app and its content. Is there a way to load in the Mail app (or any other Apple app) the content of the Librairy/Mail folder and then export in EML the found emails?

So you have a wad of mail-related data from some other iMac running macOS, and want to unpack that on something else; on an iPhone running iOS or an iPad and iPadOS, or on Linux, or BSD, or (how did I forget?) Windows, or whatever else?

How big is your app development budget, because it’d undoubtedly going to be far cheaper to rent a Mac for a week or month or so or whatever is on offer, and unpack and export there than to reverse-engineer macOS mail somewhere else. MacStadium, Amazon EC2, and some other vendors rent macOS access, for instance.

Chances of finding an existing iOS or iPadOS app that will unpack macOS mail backup on iPhone or iPad is approximately zilch, of course.

How big is your development budget?

Cheaper: borrow or rent a Mac.

Otherwise, get to work reverse-engineering whichever Mail app internals are present in that backup.

Jonathan Levin’s KOS Internals book might have some info, haven’t checked to ToC of the userland volume.

TCambazard wrote:

Export from where? I have an iOS (an iMac)'s content, with the tree structure but I am not on the iMac itself with an access to the Mail app and its content. Is there a way to load in the Mail app (or any other Apple app) the content of the Librairy/Mail folder and then export in EML the found emails?

OK. What? Originally, you said, "I am trying to understand how emails and attachments are stored on my Mac." That's a pretty unambiguous statement - "my Mac". That implies you have a Mac.

Now you are referring to something that is "an iOS (an iMac)'s content"? You have "an image of a Mac", not an actual Mac. You have "Windows, iOS and Linux machines".

It sounds like you have left a whole lot out of this. What is "an image"? Where did you get it? Are these even your e-mails?

All of that is internal information about Apple Mail. Nobody outside of Apple can answer those questions. Nobody inside Apple ever would. Why do you want to know?