How to migrate Mail between Macs without Migration Assistant?

Quit Mail.

Go to Finder and press Shift-Command-G. Copy and paste this location: ~/Library and press Return. 

Copy the whole Mail folder over.

~/Library/Preferences/com.apple.mail-shared.plist

~/Library/Preferences/com.apple.mail.plist

~/Library/Containers/com.apple.MailServiceAgent

~ tilde indicates  your user library folder

Apple hid the Users' Library folders...

Method 1:

1. From the Finder, select the Go menu at top of the screen, and choose Go to Folder.
2. In the window that opens, enter ~/Library, and click Go.

Or…

 Use the Command-Shift-. 

Start Mail on the newer one & Make sure it's all there.

zip (right-click > Compress...) the ~/Library/Mail folder and move that Mail.zip to another account.

Then in another account (if that account has no mail I often first delete the existing ~/Library/Mail folder there) uncompress that .zip by double-clicking it to the Desktop and:

Mail.app > File > Import Mailboxes... > Apple Mail > choose that uncompressed Mail folder > choose all Items.

On the left-hand-side under "On My Mac" > Import, ..., look for and move "Local" and all mailboxes inside it up under "On My Mac". Right-click and you might delete "Import" folder (it has gmail mailboxes that are online). YMMV.

...

A worse option is to in Apple Mail > Mailbox > Export Mailbox... But for some weird reason Mail.app can then export only up to 2.15 GB mailboxes so old messages are silently ignored!

Monoloid wrote:

Thanks, but it doesn't work. Meaning, I know how to get all my actual emails over, but I also need all my settings and accounts and everything just as it was on my old Mac.

Add online mail accounts like gmail in macOS Settings > Internet Accounts. Delete and re-create an account if needed. Then configure Mail.app settings as desired.

~/Library/Containers/com.apple.mail/Data/Library/Preferences/com.apple.mail.plist

~Library/Group Containers/group.com.apple.mail/Library/Preferences/group.com.apple.mail.plist

~/Library/Containers/com.apple.mail/Data/Library/SyncedPreferences/com.apple.kvs/ChangeTokens/NoEncryption/Mail/com.apple.mail

~/Library/Containers/com.apple.mail/Data/Library/SyncedPreferences/com.apple.kvs/ChangeTokens/NoEncryption/Mail/com.apple.mail.threadinfo

~/Library/Containers/com.apple.mail/Data/Library/SyncedPreferences/com.apple.kvs/ChangeTokens/NoEncryption/Mail/com.apple.mail.listUnsubscribeInfo

~Library/Containers/com.apple.mail/Data/Library/SyncedPreferences/com.apple.kvs/ChangeTokens/NoEncryption/Mail/com.apple.mail.vipsenders

~/Library/Containers/com.apple.mail/Data/Library/SyncedPreferences/com.apple.kvs/ChangeTokens/NoEncryption/Mail/com.apple.mail.remindMe

~Library/Containers/com.apple.mail/Data/Library/SyncedPreferences/com.apple.kvs/ChangeTokens/EndToEndEncryption/Mail

Folder... ~/Library/Containers/com.apple.mail/Data/Library/SyncedPreferences/com.apple.kvs/ChangeTokens/EndToEndEncryption/Mail

Folder...

~/Library/Containers/com.apple.mail/Data/Library/SyncedPreferences/com.apple.kvs/ChangeTokens/NoEncryption/Mail

Then you’re in the wrong Library folder. The tilde ~ indicates your username library, not the system library.

Oh wait, mine might be a holdover from updating a previous OS! :(

If you've copied all the others over try opening Mail on the new one.

Good idea.:)