Judging by the message you're getting they're just running it through SpamAssassin, which has an entry for that specific warning on their wiki page (archive.org)
and explains it to be because of
The message includes HTML with obfuscated text, such as unnecessary hex-encoding of ASCII characters. This is probably an attempt to avoid text-based filters
While I can't see any reason for that to be triggered by the part of your e-mail that you pasted, maybe there's something about it in the parts that were omitted? Hope this helps.
EDIT AFTER ORIGINAL POST WAS EDITED FOR ADDITIONAL INFO:
the =09 characters you're seeing represent tabs in ascii.
Generally speaking html emails are somewhat of a challenging beast by itself and most of the knowledge I could contribute are way outside the scope of this specific question, but to provide some general additional help, which i hope will be applicable to your situation:
for maximum compatibility always use the following header:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head><title></title><meta http-equiv="Content-Type" content="text/html; charset=utf-8" /><meta name="viewport" content="width=device-width" />
This is because some email clients and/or providers in their web interface will actually strip your doctype and replace them with the one you see above - this way you optimize for it. The meta tag in the head forces the encoding to utf-8 which is always helpful.
Try to not have any whitespace, at least, within your tags - as stated above the =09
you're seeing are due to the tabs you're using.
If you need a separation of paragraphs, close the </p>
and open a new one.
For example:
<p>lorem ipsum whateverrrrrrrrr</p>
<br>
<p>another paragraph</p>
Don't try to simulate vertical spacing with whitespace, you'll run into tons of trouble.
(Don't use <br>
within <p>
's either to force a break unless you want gmail to make the following text purple [this is not a joke])