This is a highly technical area of law, and arguing about it is very profitable for lawyers. The specifics are different in different jurisdictions and often in different areas of law within the same jurisdiction. So, for your example, there are probably specific service requirements within whatever Act governs energy retailers.
However, some generalisations can be made. In almost all cases, if you can prove that a document came into someone's possession, they will have been served. Then, there are provisions about deemed service, for example, from the UK's Civil Procedure Rules for service on an individual:
- Personal service by leaving it with the individual, say by dropping it at their feet,
- By first-class post, document exchange or other service which provides next-day delivery,
- Leaving it with the person's solicitor,
- Leaving it at an address that the person has nominated for service,
- Leaving it at their last known residence,
- Fax or other means of electronic communication (where the fax number or electronic address has previously been provided),
- Any other methods ordered by the court - in other jurisdictions, service by Facebook post has been accepted, AFAIK this has not (yet) happened in the UK.
It's important to note that a contract, such as one between a customer and a power company, can significantly impact the service methods. This contract may either limit or expand the available service methods, thereby influencing the legal obligations of the parties involved.
Now, here's the rub. The neighbour has nominated (incorrectly) this address, which means that the neighbour was validly served and what Scottish Power did was perfectly legal. If they had come and knocked down the door, the owner's only cause of action would be for the neighbour's negligence - not for anything Scottish Power did. Simply returning the mail doesn't change that.
And here's the second rub. Opening mail that isn't addressed to you is a crime.
