It depends. This is called nail and mail service. Before a process server can use this method, s/he had to make a reasonable number of efforts to personally serve you - or a person of suitable age and discretion - at you home or place of business. Assuming that was done, affixing the papers to the outside might be okay if the process server was not let inside the building because the outside door became the functional equivalent of your door. There is case law on this and it is often very fact specific.
I am a former federal and State prosecutor and now handle criminal defense and personal injury/civil rights cases. Feel free to check out my web site and contact me at (212) 385-8015 or via email at Eric@RothsteinLawNY.com. The above answer is for informational purposes only and not meant as legal advice.
You've been served with a summons, so the first thing you need to know is that you're now a named defendant in a lawsuit. Most likely, the summons was served with a complaint. If that's true, then you have three options: (1) do nothing, in which case the plaintiff can take a default judgment against you; (2) make a motion to dismiss the case, either for improper service or for some other reason; or (3) file an answer, a formal pleading in which you admit, deny, or deny knowledge or information sufficient to admit or deny (basically say "I don't know so I can't admit or deny") in response to each of the numbered paragraphs in the complaint.
As for your objection based on improper service, it's possible that the service was valid, but as my colleague indicated, the rules governing service are very fact-specific and hard to apply here without knowing exactly what it is that the plaintiff claims happened when process was served. One odd rule in the law, but it's very well-settled in both the state and federal courts, is that even if you have actual notice of a lawsuit, you still can move to dismiss the case on the grounds that service was improper. So don't worry about touching the summons, taking it down, sending it to your attorney, and going to court -- you still can move to dismiss the case due to improper service. However, you can't wait until trial to say that service was improper. You have to make the dismissal motion on that ground by no later than 60 days after you file an answer, at the very latest.
What you should know here is that the plaintiff's claims in the lawsuit, whatever they are, will not go away even if you manage to get this particular lawsuit dismissed for improper service. A dismissal on that ground is "without prejudice", meaning that the plaintiff can start over again with a new lawsuit and try serving you again. In other words, you are in this lawsuit now and you need to defend it, or else someone is going to get a judgment entered against you. I strongly recommend taking the summons down, reading it carefully, and contacting a litigation attorney who can help you defend and maybe knock out this lawsuit "with prejudice" because the alternative could be very expensive and even devastating for you depending on whatever is being claimed in the lawsuit. A litigation attorney also can give you advice on whether it makes sense, given your specific circumstances, to pay legal fees on a motion to dismiss based on improper service. This usually depends not only on the precise details of how the summons was served, but also on your financial/business situation at the moment and whether it's worth it to pay the legal fees to buy yourself a few months in order to litigate over a technical service-related issue.
Sorry you've been sued. My recommendation is to get a litigation/trial attorney. Good luck!
If you ignore it, then the defendant will most likely file a default judgment against you. I suggest you hire an attorney so that the attorney would file an answer with a defense of improper service or file a pre answer motion for improper service. Don't ignore it because you feel you were served improperly.