How to fix El Capitan Mail problems
Fix iCloud account
If you can’t send email using your iCloud account after upgrading to El Capitan then following these steps may help:
1.
Open System Preferences>iCloud and unclick the checkbox beside Mail to disable it.
2.
Once this process completes, click on the box to turn Mail back on, and close System Preferences.
3.
Launch Mail and open Preferences>Accounts. Choose iCloud.
4.
Set the ‘Outgoing Mail Server’ section to the right to iCloud.
5.
Close Preferences and launch Mail.
Fix Account 1.
Look at the list and at the top of the list you’ll see the ‘Automatically detect and maintain account settings’ tick box.
2.
If it is unchecked, check it and relaunch Mail; if it's checked, then uncheck it, quit and relaunch Mail, reenable it and then quit and relaunch Mail once again.
.You
should repeat the process for each of your troublesome accounts.
Disable Mail Extensions
Disable your extensions and relaunch the application.
Re-enable your extensions one at a time so you can identify which one is causing your problem. Then write to the developer and ask for help.
Stop the freeze 
If your Mail has been crashing on launch, you can sometimes prevent this behavior by taking control of your accounts and making them turn on one at a time, instead of all at once.
Contact Apple Customer Support
If you’ve tried all these and nothing has worked, then it may be time to call AppleCustomer Service.us– they’re usually pretty helpful.
Reference:- http:// www.computerworld.com/article/2991340/apple-mac/a-quick-guide-to-fixing-el-capitan-mail-problems.html
Contact US http://applecustomerservice.us/apple-email-support /
800-786-0581 support@applecustomerservice.us