Hello rkaufmann87, My iMac(24-inch, Early 2009) is running version 10.11.6 El Capitan. Apple Mail version 9.3 (3124). The problem is my Apple Mail app will start fine and stay running. I can use my other internet Yahoo email account just fine. While in the app if I click on my iCloud inbox mail will CRASH almost immediately. The iCloud mail is receiving mail, I just can't get too it. I have tried going to library folder "Library/Containers/com.apple.mail and moving that folder to desktop. Restart, re-launch Mail, but problem still there. Repeated moving Containers/com.apple.MailServiceAgent ,but still no go. I've also tried to a Time Capsule restore of the two Library/Containers files, but that was no help either. Looking at the Community Blog most people have a problem crashing on startup. Mine crashes when I click on the iCloud inbox. The first couple of lines in error message that pops up reads as follows;
Process: Mail [2106]
Path: /Applications/Mail.app/Contents/MacOS/Mail
Identifier: com.apple.mail
Version: 9.3 (3124)
Build Info: Mail-31240000000000000~2
Code Type: X86-64 (Native)
Parent Process: ??? [1]
Responsible: Mail [2106]
User ID: 501
Any assistance appreciated.