Input:

Subject:=?utf-8?B?44CQIFdpY2h0aWdlIEluZm9ybWF0aW9uZW7jgJE=?=

Take the string from the 2nd “?” to the last “?” - in this case

44CQIFdpY2h0aWdlIEluZm9ybWF0aW9uZW7jgJE=

and paste it into https://www.base64decode.org/ - Hit Decode

Result displayed in lower textbox