In some states posting on the door is legal service (possibly after personal service has been attempted to no avail). You can goggle the state laws of your state to find out what the laws are regarding Notice/Process and see if they have followed correctly. If not, those will be your grounds (standing) to file a Motion to Set Aside Default Judgment and this must be done within a certain time period, usually 30 days but may be less in your state.
Obviously, the service was improper in that you did not receive the Service, however, the burden of proof will now be upon you to prove that you did not receive proper service. If you file a Motion to Set Aside Default Judgment either pro se (on your own) or thru an attorney and you get a hearing (which you probably will) you will have to convince the Judge that you did not get the service and the other side probably will have a process server at the hearing to testify that he/she posted the papers/service on your door, in addition, he/she (the process server) has probably already signed an Affidavit of Process stating that he duly posted the papers on your door. So, I'm just giving you a heads up of the types of tricks the other side may play on you. Some attorneys have idiots like that that work for them.
Standard Courses of Action:
1. Motion to Set Aside Default Judgment
2. Appeal (this also has short time limits).
3. Settle the matter out of court.
Hope this helps.