I have been using the same yahoo mail account for about 9 years and all the emails I received were stored there perfectly. My 2009 Macbook stored every email as well. After setting up my email account on a new iPhone on the Mail app in 2014, the app only displayed emails which I received in the past month or so. My Macbook was still storing every single email.
I recently (2016) bought a new Macbook, and I noticed that only the emails which I can see on my iPhone are shown in the Mail app on the new Macbook. Checking my mail account via browser, I saw that every mail has been deleted except for the mails on my Phone and new Macbook. All the mails are still in the inbox of my old Macbook.
In short: New iPhone deletes emails on the email server permanently so that my new Macbook cannot access them. Old Macbook still stores old emails (even those not on the server) in the inbox and just adds new emails to the old ones.
My thoughts so far: I think that it must be something wrong either in the mail preferences on the iPhone or in the yahoo server. I don't know why the old Macbook still syncs with the server and keeps the old mails.
What I have found so far: According to yahoo, they had similar cases in which apps accessing the mail account deleted old emails. And I have read that some Mac-users have experienced the same problem.
Is there some way to stop the iPhone from deleting emails? Can I maybe re-insert my old mails from my old computer into the server?
EDIT: The account uses IMAP.
Best Answer
All the devices were using the same email account. But every device except the old Macbook accessed it with IMAP, the old Macbook used POP. (I realised this very late.) This caused that the old Macbook downloaded all the emails and deleted them from the server, and that's why the other devices couldn't see the emails.
SOLUTION: Created new IMAP account in Mail, then copying all the emails from the POP account to the IMAP account. The other devices were then automatically able to access all the emails.
(For someone not knowing anything about IMAP or POP, just google the difference between them.)