Why you can't send mail

Let's suppose that you are 'Sender A' on the flow chart. You send mail as suchnsuch@important.com to client@vcn.com the same way you would any other subscriber. Your message goes out of "Mail Server B", (because that's how your IT guy set it up).

Mail Server B has no problem sending your message out. In fact, the problem doesn't start until your message gets all the way to Visionary.

Visionary: "HOLD IT! You say that you're coming from important.com, let me see about that. Who is responsible for important.com?"
important.com: "I am."
Visionary: "But this message came from Mail Server B? Can I accept this message anyway?"

If important.com uses SPF but does not use SPF correctly:

important.com: "ABSOLUTELY NOT! It must be SPAM because It wasn't sent through Mail Server A."
Visionary: "Sorry little message, I'm sending you back to where you came from."

If important.com uses SPF and uses SPF correctly:
 
important.com: "Yes, I authorize Mail Server B to send mail as important.com"
Visionary: "Alright little message, good luck getting through the spam scanner!"

If important.com does not use SPF at all:
 
important.com: "I don't mind who sends mail as important.com, I bet you see a lot of spam coming from important.com!"
Visionary: "Alright little message, good luck getting through the spam scanner!"



So what can I do to fix it?

There are two options aside from giving up and not sending mail as important.com.

1. If you're not a tech and don't know much about your website, contact your website administrator, or someone who knows about your website. Whoever maintains DNS for important.com needs to update the SPF record for the domain to allow Mail Server B to send mail as important.com.
   
   
2. With some additional information provided by important.com, you can change your mail server settings in your mail program to send out mail through the intended server without making changes to the SPF record, this would be a quick resolution if your website administrator is on vacation.

Hopefully you can see why SPF is a good thing. Even though it wasn't intended to do so, it could eliminate a lot of spam from the Internet.

For more information on creating a valid SPF record for your domain, check out the following resources

Microsoft SPF Record Wizard
OpenSPF