Susan, the point you don't seem to understand is that asking for an e-mail to be double-spaced is completely and utterly pointless because there's no way to guarantee you'll get what you asked for. So those of us who understand e-mail are trying to point out that the agent is probably NOT asking for the e-mail itself to be double-spaced. E-mail is, quite simply, impossible to control.
I know that there are people here who say you can control it, but they're wrong: you just can't.
Since a picture says a thousand words, here are some. My e-mail client doesn't offer a line-spacing setting, so I composed the following e-mail and manually entered two returns at a place that looked about right:
Here's what the e-mail looked like in the
very same e-mail client upon receipt:
And here's what it looked like in my host's web-interface client:
This is why we're saying not to double-space sent mail. If the recipient wants to read it double-spaced, they'll copy-and-paste the material into a word-processing document and reformat it.
This is in no way comparable to the agent who wanted a PDF in TNR 18, because the PDF will look the same to both sender and recipient. E-mail is NOT guaranteed to look right.