URGENT:Must delete my alias but still access that old email in/out in past.

Question

I MUST immediately no longer send or receive email from an alias email name i have set up on my dslextreme ISP.
I know i can do that if i go onto that ISP and 'delete' that alias. Then that alias will no longer be used to either send or receive, they said. OK.
But definitely want and need to still be able to see all the old email I have ever sent or received (over the years) in the past.
Doesn't seem right that it might be lost, all my old in and out records, and not viewable ever again in my 'MAIL' aplic.
I want to still be able to access and read all my old in &/or out mail.
My ISP tech support said I could delete that alias on their end, but if I maintained it on my Apple 'MAIL' it would be constantly searching for an inactive address and sending me messages every day that it was unable to find that account. That's okk, I guess, now that I think about it. I just worry that I must somehow still be able too read all my old past in and out letters. If I can do that I'll be happy.
Doesn't seeem right that if I make an email address deleted or inactive on MAIL Prefs, that I can never see those old messages again.
PLEAse help. Need to do it immediately.
Thanks

Answer 1

If the account was a POP type account, your emails should already be stored locally. If the account was an IMAP type account, then it is saved on the server, so if you will lose access to the server, you should copy them locally. If you don't know, you should be able to tell by opening Mail preferences to the Accounts pane. Select the account in the sidebar and look under +Account Information+. The +Account Type+ should be shown there.
If it is IMAP, you can copy your email to your local drive.
Look to the bottom of the Mail window sidebar. You should see a plus sign. Click that and select +New Mailbox+. In the drop down dialog box, for Location select *On My Mac* and name it. A good name might be "email from (alias)" or something that refers to that alias. You should see a new +On My Mac+ section appear in the Mail window sidebar, if you did not have one before.
Repeat the plus sign +New Mailbox+ command again and again, to create folders (mailboxes) within your previously created folder. Now copy your email as you see fit from your existing account email folders to your +On My Mac+ folders.