Resolving Mail Problems After OS X 10.10.4 Update
Some Mac Mail users have discovered that Mail app misbehaves after updating to OS X 10.10.4 (and some with OS X 10.11 El Capitan) particularly with Exchange and Gmail accounts, but it can happen with other email provider services as well. Typically the problem manifests in one of a few ways; eMail logins suddenly stall or fail, Mail app fails to check for new email, Mail app fails to send emails, or Mail app just doesn’t do anything at all and will basically hang on launch as a never-ending server connection is attempted but fails to establish.