It's hard to see what's happening from your screen capture.
Open it in Acrobat Pro. Go into editing mode, and show the form so it shows the size of the form fields like this one:
There is no control for vertical alignment in InDesign or in Acrobat Pro. By default, text is centered vertically. But it's hard to see if we can't see the dimensions of the form fields.
It's strange because again, (after exporting the form to PDF Interactive) I filled the pdf form in with my name, address etc and highlited the fields. At this point the text is centered. I then clicked submit.
It went to my email, but looking at it the pdf attachement within my Apple Mail the text aligns at the top. If I click on the attached pdf it opens in Acrobat X and the text is correctly centered.
I don't think this has anything to do with Apple Mail because I sent the Interactive pdf form to a friend. When he began to fill the form he saw the text in all text fields to be too high.
Here is a screen shot of the attached pdf within my Mail program (cannot show the highlighted text fields) and the same pdf on the Desktop with highlighted fields.
Your friend probably viewed the PDF in Apple Preview. Apple Preview is notoriously lousy at handling PDF forms. Apple Mail uses, I think, the same rendering engine.
You need to view in Adobe Reader or Acrobat to know for sure.