Change to noreply address for marketing sendouts

Hi, 

Scenario:
I want to change from the standard email-address used for sendouts (mail-box adress set up fo a que) to a noreply@company.com adress.

We have a spcecial setup using CRM-script and  an reply-template so it is not har to do the change.

What I realy want to know how the impact of sending with this new noreply@company.com if:

1. noreply@company.com acutally does not exist in the company.com domain email-server
2. noreply@company.com exist in the company.com domain email-server on a shared o dedicated mailbox.

Eg will a mail be more likely to be tagged as SPAM if mail-adress/mailbox does not exist when we use that as sender-adress?

Does anyone have clear anwser here?

//Anders