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There could be a couple of reasons for that. The main reason is that not all image viewers support the same SVG standards. And especially, an email program's preview will not have anything but the very most basic image displaying ability.
In addition to flowed text, these are some other features which we are commonly not displayed in previews (file manager previews....well, file managers period, often won't display these things from SVG files, either). Filters, patterns, markers, possibly results from various extensions, maybe transparency (full or partial) can't be displayed properly....and there might be something else that doesn't come to my mind at the moment.
Another possible reason for this might be that not all the contents of your file are inside the page border.
I can't say for certain without some testing, but almost certainly, the person on the other end is receiving the file, just as you saved it. Well, assuming they have the proper program to display it, whatever it is. If they don't see it correctly in their preview, try and urge them to open it properly, before concluding there's a problem with it.
"Properly" means for an SVG file, open it with Inkscape. Browsers can probably display SVG files the best, but they can't always display all types of contents, which Inkscape can create.
