Authenticated SMTP Server
Have you ever been denied from sending email while 'on the move'?
This happens when you use your company / ISP smtp server to send email while not at the address they expect.
tech bit: Your IP Address is an address given to your PC / Network to identify it on the internet. When you send an email you must first connect to your smtp server. This server checks your address to make sure you are 'authorised' to send email then passes your email to the intended recipient email server. The most common way to check your computer is authorised is by IP address, which is different in different locations! (Work, home, friends / family, free WiFi will ALL have different addresses and may stop you sending email)