In Colorado, a criminal subpoena must be personally served or get a valid waiver of service to be a valid court order. This means that the server (usually a sheriff deputy) personally identifies the subject and attempts to give them the document. It does not have the touch the person and the person does not have to take it. The server must have a good faith reason to believe the person they are looking at is the subject of the subpoena. They usually just ask for the subject and if the subject says, "that's me," they hand them the document.
A mailed subpoena can become a valid court order if the waiver of service is signed and mailed back. When a subpoena is mailed, the letter makes it seem like there is a mandatory requirement to sign the waive and mail it back. This is not the case. You do not have to mail it back or waive personal service. BUT, if you sign it and mail it back, then it is a valid court order with contempt of court consequences.
In Boulder and Longmont, if a Domestic Violence DV victim tries to get the case dismissed, the prosecution will personally serve a subpoena on the Domestic Violence DV victim to make them come to court (rather than mail a subpoena). In Adams county, the police will often subpoena the Domestic Violence DV victim the same night as the arrest.
FYI - there is never a requirement to answer your door. If you do not answer your door for anyone that you do not already know, then you will not get subpoenaed when you do not want to be. But, beware, you are not allowed to lie to the police. You do not have to answer their questions, but you are not allowed to lie.