As my “old” one was acting a bit flaky, I did this one as a clean install rather than use Migration Assistant, and have been adding applications as I need them and find I have not installed them.
Most of my email is in IMAP accounts and so has downloaded automatically once I set the account up in Apple’s Mail.app. However, I also had some old emails stored in the “On My Mac” mailbox rather than in icloud or exchange.
These are mostly completed projects which I am unlikely to have to access, but need to retain just in case, for record keeping purposes.
I am wondering how I can find and transfer the “On My Mac” mailboxes over - either to my new computer or, perhaps even better, to my Synology NAS?
Why not copy the “On My Mac” mail items (or whole folder) over to an IMAP account, then on the new machine either keep them there, or copy (then delete) the mail items from the IMAP account to the “on My Mac” on the new machine.
As for copying to a file server (NAS), there are lots of utilities out there to assist with that. I used to routinely use the capability in DEVONthink to do that sort of archiving, but in recent years I haven’t bothered doing it and am keeping the mail in local or IMAP mail servers. Inertia, I guess.
However, there are a number of “folders” with alphanumeric codes in there which each have numerous mailboxes in them - none helpfully labelled “On my Mac” and many having names which line up with the IMAP accounts already transferred / downloaded.
If you want to archive these just for possible future reference why not create a PDF of them? As PDF docs they’ll be easily searchable. If you have a ton you can select specific date ranges and break them up that way.
Highlight the desired email and then under File select Export as PDF…
1 You can export the folders to mbox files then re-import them. As I recall, you have to do this to each folder individually. You cannot export nested folders all at once.
Or you can select the contents of an email folder then drag a copy of the messages from mail to a normal finder folder on your Mac or to remote (NAS) folder. The messages are searchable by spotlight and will be opened by mail.app when double clicked and handled normally if you want to reply, forward, etc.
Like FrankT I would suggest converting them to PDF. Mail systems make very poor filing systems. I have seen too many people lose their emails when the mail database gets damaged.
EagleFiler is a solid option for archiving, and it works with standard mbox format, or as individual messages (which takes up more space, and slows down the system with large archives).
"EagleFiler prefers to store messages in mailbox (“mbox”) files. A mailbox is a single file that contains lots of e-mail messages. This is both for efficiency (one file rather than thousands for a mailbox containing thousands of messages) and for compatibility with other software that can read the standard mbox format. When you capture multiple e-mails at once, EagleFiler stores them in a mailbox file automatically. You cannot drag message in mailbox files around to different mailboxes, though you can drag them to tags and organize them that way. EagleFiler can also store messages as individual .eml message files, one per message. This is less efficient (for EagleFiler, Spotlight, Time Machine, etc.) but is more flexible because you can drag and drop the messages to different folders to rearrange them. Message files can also be mixed in the same folder with other files of different types, e.g. PDF or Word documents that are related to those messages. You can double-click a message file to open it in Apple Mail."