Mirror HTTP port shouldn't normally be changed (that's the port number used by the mirror HTTP server) unless you have a good reason to do so (e.g. conflict with another application that's using port 5033).
POP3 server, username and pwd is the PUSH-updating thing.
Here is some introduction to the PUSH updates:
http://www.avast.com/i_idt_152.html#7In case of ADNM, you should have a special mailbox that will be used exclusively for the PUSH updates. It can be either on your own mailserver or any other (e.g. hotmail) - the only requirement is that it be accessible via the POP3 protocol.
You then subscribe the iAVS service to that mailbox --
http://www.avast.com/i_idt_112.html and enter the mailbox details to that mirror settings in the Console. This will force the AMS to pop new messages from the mail server and interpret the iAVS mails as updating triggers.
Hope this clears it a bit.