<p><strong>Duplicate of <a href="https://stackoverflow.com/questions/371/how-do-you-make-sure-email-you-send-programmatically-is-not-automatically-marke">How do you make sure email you send programmatically is not automatically marked as spam?</a></strong></p>

<p>Initially I planned to send emails from my website as html (i.e. content-type: text/html;) but I just thought about something today, won't it better to send emails as plain text, i.e. content-type: text/plain;, so that the chances of my mail messages ending up in the spam/junk folder become less, I was thinking that this must be one the rules used by spam filters on popular webmail services like gmail, yahoo and hotmail, what do you think?</p>

In my experience, most anti-spam systems will score HTML's Hamminess slightly below plaintext, but there are much more important things to consider that affect deliverability:
make sure you have a valid Reverse DNS configuration for your mail server, or you will be undeliverable to many domains.
investigate adding an
SPF
record to your DNS zone file. This helps, particularly with GMail.
Look into
DomainKeys
, if it's available on your platform. This is essentially a cryptographic signature attached to your outbound mail, so it proves that you are the person who sent it.
Review your message content and make sure you aren't using any words (particularly in the subject line) that are obvious spam indicators.