You should allot a certain portion of your day to email and to regular mail. For example, allot one hour in the morning and one hour in the afternoon to handling all your written communications. Unless you dedicate specific time slots to the handling of email and post, you will soon find yourself on the downward slope of decreased productivity as you constantly check and respond to email to put off whatever you should be doing.
In a nutshell:
In the end, the success of your VA business is entirely and completely reliant upon you and the decisions that you make. Success is within your reach, if only you can stay focused on your goals. You must decide to reach for your goals, and then, you must have the discipline necessary to reach them.