Trespassing: unlawful entry upon private property.
When serving process, do not trespass. It is fine to be in any area that is evidently open for the public. For example, the sidewalk is an area for public use. A path extending from a sidewalk to the door of a home or business is, again, implied to be used by the public. While approaching a home, the expectation is that you knock or ring the doorbell. When entering a business, the guidelines are the same. Anywhere before the front desk is open to the public, but crossing behind the front desk can be considered trespassing.
Being in areas that are evidently open to the public does not give you unlimited freedom, however. You may not interfere with any person's expectation of privacy from an area of public use. This means not looking through or knocking on windows, not touching mailboxes, or attempting to find an alternate entrance to someone's property besides the most obvious. Similarly roads and sidewalks are public spaces, but it is not acceptable to block someone's driveway.
If you see a "No Trespassing" sign in front of, or next to, a residence, then please respect the homeowner's wishes. You should instead log an attempt in ABC Mobile specifying why you are unable to knock on the door. Then, place a phone call to the servee using the phone numbers provided to you and try to get them to meet you for service.
A "No Trespassing" sign is not the only thing that should keep you out of certain areas. It is not okay to go anywhere that is implied to be closed to the public. This includes areas locked off by a fence, backyards, or areas behind the front desk of a business.
A good rule of thumb is to ask yourself, "If I owned this property, what areas would I be comfortable with a stranger occupying, and what actions would I be comfortable with this stranger taking?"
Always remember that your own safety comes first in the matters of considering whether a property is accessible. Be mindful of how your actions could be seen by the residents of the home, and err on the side of caution.