I recently upgraded a customers' operating system from Macintosh Yosemite which is 10.10 to El Capitan which is 10.11. Everything went smoothly, until the customer could NOT use their Apple mail. Here is their response after this upgrade:
"Aaron, I know you are sleep now, but when you are back at your computer please let me know how to get my e-mail to work. Since the El Capitan was installed, I can't get my e-mail to respond at all. When you click on mail it is completely blank.
Here is my response to this perplexing situation. Make sure that Apple mail is not opened, before changing these settings. In the dock, Apple mail should not be highlighted.
You could reboot your MAC to make sure that Apple mail is not opened. Try the following steps. Quit Apple mail.
Open "System Preferences" then choose "Internet Accounts". Click on the email account you are having trouble with and turn off "Mail". Turn "Mail" back on again. Now open Apple mail.
Initially, this did NOT fix this Apple mail problem. However, I think the customer must have performed the proceeding steps a second time. They were then able to successfully access their email.
"The things you told me must have worked. I'm getting both of the e-mails now. You always know just what to do. Thank you. What state are you in?"
Operating system upgrades are NOT always a picture perfect venture. That includes Macintosh operating system upgrades. Thankfully, only one issue was caused by this Yosemite to El Capitan upgrade.
However, not being able to access email might be a major crisis for some Macintosh users. Also, believe it or not this was the first solution I presented to this customer. It may appear as if I have all the answers for computer problems.
I researched this problem and tried to target it to problems after Macintosh operating system upgrades. I do NOT always fix problems with the first solution presented. However, when I do fix problems the first go around, it validates perhaps my use of logic.