Added a permission and changed commands format a little.
Permission mail.admin allows a player to use the /sendmail command. Mail can still be sent via mailbox without a permission.

Command /sendmail now uses true/false instead of 0/1 to specify whether the virtual mail should be sent anonymously or should display the sender's username.

The help message sent if the /sendmail command is entered incorrectly is now grey instead of white. Custom colors will come later.

The Java compiler version has been updated (downdated?) from JDK14 to JDK8 for greater compatibility.