In our Server we've implemented SPF and opendkim. The IP of our server is 188.8.131.52. SPF and opendkim verification works as expected.
The problem is with one of our users. I don't know the reason, but when receiving emails from them to my personal gmail account (for testing purposes), gmail throws me the following error if I check the mail headers:
Received-SPF: fail (google.com: domain of email@example.com does not designate 184.108.40.206 as permitted sender) client-ip=220.127.116.11;
You can see the message wasn't sent using our server. That IP is just the public IP of my client. His email client is Outlook 2003 (Windows 7). His configuration is fairly simple: there are two computers: one of them is a normal computer; the other one is Windows 7 whose hostname is SERVER (SERVIDOR in Spanish). However, his installed OS is not a server version of windows, but a common one (without any SMTP server installed as far I know). It doesn't matter the computer from which the message was sent; in both cases, the client IP is the same.
What can be the reason? Which type of test should I make? Proxy issues? In the latter case, how can I know if they are using a proxy?