Non gov emails could be blocked but they don’t. They do use spam blockers, but that usually results in items being sent to a “junk” folder, which can still be accessed.
So, shortly after taking office it is very likely that Hillary had to follow-up her e-mails to .gov, including ones to 0bama, with a phone call saying “I sent you an email?,,, Check your spam folder”
“Check your spam folder”
You can set your spam blocker to block emails from domains or individual senders if you want, but they generally block based on keywords like “viagra” in the subject line or in the content.