Since uploading to El Cap if find my apple Mail frequently locks up when I toggle between Mail and Safari. I have to quit and launch mail to clear the fault. Also a much larger number of emails appear as code and I have to go to Safari to read content.
I have read mail quits unexpectedly link but no help. All software appears to be up to date and I do not think I have any plug ins and I don't have a library/mail/bundles folder to delete.
iMac mid 2011 27inch EL Cap 10.11.1, Safari 9.01, Mail 9.1