The general rule is that when spousal support is ordered by a court, it is deductible by the person who pays, and taxable to the recipient. There are rules that apply, but they are relatively straight forward.
Occasionally, the recipient is a non-resident alien who decides to live in another country: Maybe they return from their country of origin, just want to get away, or they have family there - maybe they can make their support stretch farther. Some who have been married to American citizens for decades were here legally, but remained citizens of another country, and when the marriage failed decided to go to their homeland.
The taxability scheme assumes the government gets tax from one spouse or the other. Although the recipient is usually in a lower tax bracket, that is a bargain we make. When the receiving non-resident spouse moves to Italy, Poland, Japan, or Canada, for example, how does the government get its money? To often, no one asks that question, and finds out too late to solve problems created.
Well, there is an answer: The paying spouse withholds the money and sends it to the IRS. This does not apply in all cases, as there some inter-country treaties that eliminate the obligation. The risk to the paying spouse is that failing to withhold may make him or her liable for the tax that should have been paid.
Will the "document preparer" helping you fill out the paperwork to process your divorce have any clue? Of course not. Even most lawyers don't know the rule unless they attend the type of courses that teach these unusual rules - certified specialists are more likely to take such courses.