I'm using Yosemite at the moment; thinking of moving to El Capitan. One factor is what happens to my mail accounts because I have thousands of emails on my mail server meaning that a clean install of the mail accounts is very time-consuming as it downloads every single message ... Any advice much appreciated
imac, Mac OS X (10.5.4)
The upgrade will not affect your Mail messages.
If it is your intention to erase your Mac, read Erase a drive and install OS X. It includes instructions to back up your important files first. Do that. Then, perform the upgrade, erase your iMac's startup volume, install El Capitan from the Recovery partition, and finally migrate your User account data using Setup Assistant. It will restore your Mail messages.
Thanks John. Is it possible for me to upgrade to El Capitan without erasing the mac and if so will the upgrade affect my mail accounts? I'm not sure which to do
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Yes, in fact that would be considered the normal way of upgrading OS X. There is no need to erase your Mac. Your Mail accounts and their contents will be unaffected.
Backups are a good idea in general though. You should back up your Mac routinely, even if you were not contemplating an upgrade. To learn how to use Time Machine read Use Time Machine to back up or restore your Mac - Apple Support. All you need to purchase is an external hard disk drive of adequate capacity. A 1 TB hard disk drive in a USB enclosure costs about $50.
Upgrading to el Capitan - question about mail accounts
