Definitions for "Welcome message"
an automatic message that will go out to your users when they sign up for your newsletter through your Opt In Form on your website
an autoresponder - a message that's sent to new names when they are added to a list
an email sent to all new users who subscribe to your list, and a Goodbye message is an email sent to all users who unsubscribe from your list
a convenient way to transmit information of general interest to all users of the system
a useful way of indicating to users that a Web site is personalized, but if there is no real personalization, it's just a waste of space that I personally find irritating