Spam filters depend on the content of the email as opposed to the programming that goes behind sending the email:
Spam filters look at a long list of criteria to decide whether your
email is junk. They might look for spammy phrases like “CLICK HERE!”
or “FREE! BUY NOW!” They’ll assign points every time they see one of
those phrases. Certain criteria get more points than others. Here’s a
sample of criteria from SpamAssassin
Talks about lots of money (.193 points) Describes some sort of
breakthrough (.232 points) Looks like mortgage pitch (.297 points)
Contains urgent matter (.288 points) Money back guarantee (2.051
points) If your campaign’s total “spam score” exceeds a certain
threshold, then your email goes to the junk folder. You’re probably
thinking, “What’s the threshold I need to stay under?” Sorry, but the
number is different for every server.
As for that list of “spammy” criteria, it’s constantly growing and
adapting, because spam filters learn more about junk every time
someone clicks the spam button in their email program.
Spam filters even sync up with each other to share what they’ve
learned. Although there's no magic formula, we can help you avoid
common mistakes that send newsletters to junk folders.
Avoid these common mistakes
These are the most common mistakes we see new email marketers make,
which result in accidental spam filtering:
Using spammy phrases, like “Click here!” or “Once in a lifetime
opportunity!” Going crazy with exclamation points!!!!!!!!!!!!!!! USING
ALL CAPS, WHICH IS LIKE SCREAMING AT THE TOP OF YOUR LUNGS VIA EMAIL
(especially in the subject line) Coloring fonts bright red or green
Coding sloppy HTML, usually from converting a Microsoft Word file to
HTML Creating an HTML email that’s nothing but one big image, with
little or no text (since spam filters can’t read images, they assume
you’re a spammer that’s trying to trick them) Using the word “test” in
the subject line (agencies run into this when sending drafts to
clients for approval) Sending a test to multiple recipients within the
same company (that company’s email firewall can only assume it’s a
spam attack) Designing HTML email in Word and exporting the code to
HTML (That code is sloppy, and spam filters hate it.)
Source: MailChimp: How to Avoid Spam Filters