You have to use a third-party app, on MacOS I haven't found any free ones (on Windows you have some free).
This one for example:
All messages in an .mbox mailbox are stored as plain text so you can also open it with any text editor.
There are a number of existing questions/answers that address this very topic:
Archiving mail from Mail.app
Archiving Apple Mail to an External Drive
Move mail boxes data to external drive on Mac Mail on Mavericks
Moving individual Mailbox to external drive
These posts should get you on the right track to migrating your mail over to the external disk.
To implement server side rules, your server must support it.
There’s nothing in Apple Mail that can enable something on the server if the server doesn’t support that feature.
I searched Cox’s Support site for “server side rules” and found nothing. Additionally, I found this message
Cox Email is a free service that was included with all Cox High ...
You can't remove the email functionality from a icloud.com or me.com adress. What you can do instead is to
log into icloud.com in any browser
go to Mail
click on the cog wheel at bottom left and select Settings (first entry in the menu)
set auto-forwarding of all mails to your new email adress
This is impossible as clearly stated on Apple's website:
How to change your Apple ID if it ends with @icloud.com, @me.com or @mac.com
If you use an @icloud.com, @mac.com or @me.com email address as your Apple ID, you can’t change it to a third-party email address.
That is your Apple ID so you can't change it, the only real alternative is to create a ...
This Mail button is used to have your Apple mailbox (@email@example.comfirstname.lastname@example.org) on your device. If you turn it on your Apple mailbox will just show up in the Mail app.
Note that this option does not synchronize the other mailbox you might have, so in your case it will not be useful. You can find more informations about it here.
Delete only work in some cases, check your email configuration before you act: if you're using POP3 or your IMAP settings are MOVING messages from the server into your local computer, deleting messages or attachments will result in loss of data.
I would recommend trying with the mailbox rebuild procedure suggested above.
You may need to rebuild a mailbox to update the list of messages it contains. For example, if messages appear to be missing or garbled, or if you can’t find all relevant messages when you search using the Entire Message search option.
In the Mail app on your Mac, select a mailbox in the sidebar.
Choose Mailbox > Rebuild.
When you rebuild mailboxes for ...
It is possible to choose which mail to use in Sign in with Apple :
First go to Settings > Your Apple ID > Name, Phone Number, Emails > Make sure the email you want to use is added.
Open the app and click Sign in with Apple and then click on Share my Mail.
It will then propose you which mail associated with you Apple ID you want to use.
Note that it doesn't ...
It's called an email alias.
Create or change an alias
In the Mail app on your Mac, choose Mail > Preferences, then click Accounts.
Select an account, then click Account Information.
Click the Email Address pop-up menu, choose Edit Email Addresses, then do one of ...
I tried Speaking Email (speaking email website link) and am finding it really great. It's a one by one email reader which keeps it nice and simple. It has commands like "delete", "reply" and so on which are voice commands, a nice idea - really powerful and no need to wear your eyes out peering at a screen.
The reply dictation works well. It is best for ...
I know it's a long time now that this question has been asked, but I did experience the same problem.
I solved it by removing a file located in my preferences folder, that is :
Once I removed this file (after quitting Mail.app), the STMP server choice was "sticky" again.
I know this is old but I just had this happen to me on iOS 13. I was trying to mark all email as read and slipped and marked everything as junk... After correcting that nightmare, I noticed my email showing on my phone was very old. I read the solution here about restoring the phone from the backup and I did not want to do that. So the solution I came up ...
To simply reset the account, go to Tools > Account Settings. Choose the account, go to the bottom left of the screen (Account Actions) and click "Remove Account."
Then re-add the account from scratch.
If you are using the IMAP protocol, you will not have lost any emails.
Though to be safe, you can navigate to where the data is stored and back this up ...
I had this problem for a while, but it seems, somehow, to have worked itself out (which is always worrying). To be more explicit:
The problem started showing up, very suddenly, about two weeks ago. It affected my two macOS machines, one located at my institution, the other by broadband at home. I could get the messages fine via iOS or Outlook itself (...