My boss upgraded his MacBook to OS X El Capitan, and after doing so, Apple Mail started downloading again all mail messages from the server. As he has around 130K mails stored on the server, this makes the computer crash.
Why did AppleMail forgot all about mails already downloaded when upgrading the OS? How can we fix this? We already tried unabling all accounts and enabling them one by one, but there are 130K mails in one single account, so this was no help.