Recently I was made aware by some friends that emails I was sending to their Gmail accounts were never delivered. After days of freaking out and trying to find the problem I think I have finally fixed the problem. So it turns out that I had to publish my SPF entries.
SPF (Sender Policy Framework, aka Sender Permitted From) is an email protocol used by many ISPs and aparently now by Gmail that basically tries to identify spoofed addresses. It is used as part of a series of other tests to sort out spam. Basically it adds a Return-Path: header to outgoing emails. This is then read by the receiving servers to identify whether the sender is really who she says he is.
There is a very easy to use SPF Wizard which creates a one line record that you add to your DNS zone. For example an SPF record might look like:
example.com IN TXT "v=spf1 ~all"
For more detailed information on SPF I would recommend reading about how SPF Helps Legitimate E-Mail Get through Spam Filters and also the FAQ section on SPF at POBox.