aggression

It sounds like she is just doing when she was bred to do - guard and protect.

My dobe will do the exact same thing - but once he recognizes the person at the door he will immediately stop and become the happy, social lovebug that he is. If it is a stranger or person he has never met, we make him sit and stay or lead him away from the door. Once the person is inside we have him go up to them to smell them and so they can pet him. Once he knows from us that they are ok he is fine with them.

If he is outside and someone pulls into the driveway or starts to walk up to the porch that he does not know, that is a different story. It is harder to have him calm down but it is not impossible. While I don't think that he will bite, I do not like people to just unexpectadly show up that he does not know while he is outside. He is extremely intimidating when protecting his property...but that is what Dobermans do - they guard and protect. He is not aggressive in any way but I do not want to take the chance of him hurting someone who does not "respect" him in his territory.
 
I'm not sure there is anything you can do to 'train' this behavior out of your dobe since that is what they were bred to do. That would be like trying to train a border collie or aussie not to heard or a bloodhound not to track! You probably can desensitize her so she is not as protective when people come over. Try giving guests treats before they enter the house so when they do enter they can hand her a treat. She will then learn that people mean good things. I'm sure she will still bark but maybe not as 'agressively'. If desensitizing her does not work you may want to not have her go to the door at all until you let the person in. If she thinks she is the dominant one in the pack, then she may be answering the door before you to assert herself as pack leader. Prince does bark when someone comes to the door but he never gets to the door before me. He always lets me answer it and let the person in. He is not the one to greet the guest, my husband and I do. Once they are allowed in by us he is fine.

Crock-
Prince Pride is right - it's in the breed make-up to be protective.  Now it's up to you to manage it - tougher with a dog who's been practicing her "Guard The Door At All Costs" routine for 6 years.  Do yourself a favor and talk to a Doberman-savvy trainer.  You don't want to do things that inadvertently encourage the behavior.  Things like saying "Shh!  It's ok" while holding her collar as she escalates her behavior.  Essentially, a dog may read that as - Dad likes this behavior (it's ok) and he's tensed up and ready to fight, too - he's "got my back" (holding the collar without controlling the behavior), etc.  This may not be you/your dog, but whatever your situation/scenario, there's good and bad ways to manage it.
 
A Dobe-experienced trainer (like mine with 20+ years training/handling/owning Dobes) has addressed this concern with probably two thirds of his/her Dobe clients and surely has more than one suggestion to help you manage your dog and keep you out of court over a dog bite.
 
If you need help finding a Dobe trainer, let me know where you are located and we might have some names for you.  Good luck!  Enjoy your Dobe - she will be a best friend, if you build that relationship right!