Greylisting - What is it and how does it work?
What is DNS and how does it work
How to increase the PHP file upload size limit (upload_max_filesize)
How to specify the upload_max_filesize for PHP
The Domain Exceeded The Max Defers And Failures Per Hour
Max Deferrals Limit - This tool tracks and limits outbound mail based on the maximum percentage of failed or deferred messages a domain may send per hour.
Out of Hours Emergency Procedure
What to do if you have an emergency out of normal office hours