For eg. when a System User is using Outlook and replies to an email from their own client, it reaches the Email Account with the threading info lost and the (sender + subject match) doesn't work because the sender in the first communication was someone different to whom the system user is replying to via the common email account in Frappe. This fix bypasses the sender match when the sender is a system user and subject is atleast 10 chars long (for additional safety)

Conditions involving STANDARD_USERS, removed Validate Max Users,
Cast Floats and Ints for Single DocTypes, Ability to Disable Scheduler