I use Mac-Mail since – forever. My local Mail-Archive consists of about 130'000 E-Mails from multiple accounts, stored in hundreds of folders, localy on one of my Macs. The archive goes back 18 Years! The oldest E-Mails are from 2002!

Now I finally want to have all of them available all the time from everywhere on every device I own. Therefore I want to migrate those thousands of emails to my IMAP-Accounts (3x GMail, 1x iCloud, 1x private Server; I have multiple accounts 'cause of work, honorary activity, personal etc).

Whats the most efficient way to migrate such a large local archive to a single (or better: multiple) IMAP-Account(s)?

Some notes:

• It's OK if I loose the structure of the current local archive (The folder hierarchy)
• Since many of the (older) messages are from E-Mail-Accounts not longer active or existing (f.e. old @yahoo.com Mails), it's OK if I have to move many of them to a IMAP-mailbox in another Account (f.e. to my current iCloud-Account)
• If there is no easy way to split the local Archive and move some messages to Account X and others to Account Y, it's OK if I have to move all Mails to just one IMAP-Account (yet: Gmail, iCloud, private … which should I choose?)

I thought about creating a new Smart Mailbox and filter the whole local archive for all E-Mails of OldAccount001, then move them by hand (drag-and-drop) to the «Archive»-Mailbox of my CurrentAccount001 … then edit the Smart Mailbox to filter the whole local archive for all E-Mails of OldAccount002 to the «Archive»-Mailbox of my CurrentAccount002 et cetera parge parge … this way I sure loose all the structure (folder hierarchy), but I can live with that … but will this actually work? Again: I'm speaking about thousands of locally stored E-Mails, some 18 years old, from accounts no longer existing – of course many of them with Attachments etc.