I use Auto Response Plus and was having the same problem having my emails go to the bulk folder. I added an SPF record to the sending domain and it fixed the problem.

It's a very good idea to add SPF records to your domains to prevent spoofing.

You can check to see the status of your SPF by going to DNS Stuff: DNS tools, DNS hosting tests, WHOIS, traceroute, ping, and other network and domain name tools.

Put your domain in the DNS report. Scroll down to the bottom of the report and if everything is configured properly you will see:

PASS SPF Record
You have an SPF record. This is very good, as it will help prevent spammers from abusing your domain. Your SPF record (I don't check to see if it is well designed!) is:
"v=spf1 a mx a:spf.dreamhosters.com" [TTL=14400]

Otherwise you will see another message.


The important thing to remember is what whatever domain you put as the return/reply address, you should have configured an SPF record for it. For example, if you're sending email from Comcast's network and your reply address is "joeschmoe@gmail.com", the receiving server will lookup the SPF for "gmail.com" and see that the mail server from Comcast is not part of the SPF record set up by Gmail. So it thinks that the email is being spoofed and is then flagged as junk email and put into the bulk folder.

Whatever domain name you choose to put as the reply address, be sure to have SPF records configured to have the IP address of the mail server permitted to send email with that domain.

An SPF record in your DNS tells the world which mail servers are authorized to send mail with that domain.

Also see:
SPF: Project Overview