To me it looks like an older persons handwriting - just my opinion
Or someone who's used to writing the script of arabic or farsi, perhaps? The plot seems to be thickening.
To me it looks like someone using a non-dominant hand in an attempt to confuse any handwriting analysis...weird..
Have you ever seen an older person's handwriting? My father died at 89. His handwriting -- other than while he was recovering from his TIAs -- was far better than mine. Older people were taught penmanship in school, and their handwriting -- at least in my experience -- tends to flow, and be quite graceful when compared to "younger people's" handwriting.
That said, the first thing that popped into my mind when I saw that picture is that whoever addressed it was using block letters in an attempt to foil handwriting analysis.
The second thing that popped into my mind was that perhaps whoever filled out the address used block characters so as to not foil the OCR scanners at the post offices mail sorting machines?
A person who has not mastered writing in upper and lower case would do something similar. That is a possibility. that indicates to me someone who hasn't learned English writing beyond a very early stage. Someone with perhaps 1 year of learning writing. Perhaps even self-taught. The pen strokes on the letters are not very advanced either.
If it was domestic, it is either a child of 6 or an adult who is either illiterate or doing a fine job of pretending to be illiterate. Anyone doing that fine of a job of pretending to write so primitively is much too intelligent and would have spent way too much time to just be a jokester. More analysis later.