Saxon

I attach sample2.xsl which shows how your code can be made streamable by splitting it into template rules.

The reason this works is fairly subtle. In your code, the sequential logic means that Sender, Summary, and Persons are always produced in that order, regardless what order they appear in the input. The XSLT compiler doesn't know what order they appear in the input, so it has to assume the worst, which means that the output order might not be the same as the input order, and that makes the code non-streamable. By rewriting it using template rules, my revised version outputs Sender, Summary, and Persons in whatever order they appear in the input (they might always be in that order, but the code will still work if they aren't). So the code isn't doing any re-ordering, and is therefore streamable.