It seems as though google recently made a change to how Gmail / Google Apps handles forwarded emails. A forwarded email (even within your own domain) can cause a “soft fail” in the SPF records and cause Gmail to add a warning to the email like the following:
We’ve seen this issue frequently since March 29th, the date on this support doc
If you see this error message, take a look at the headers for where the “soft fail” is happening
Previously, we haven’t noticed “softfail” causing any issues with gmail or other providers. The spec says “softfail” should have the intended action of “accept but mark”…
Unfortunately in many cases only the recipient can fix it.
If you have Google Apps for domains and you’re forwarding mail to your Gmail account, make sure that you have correctly configured SPF records for Google on your domain. Or really, however you’re forwarding email, make sure the SPF record is valid in your DNS.
Wherever you’re forwarding mail to yourself, you become the party that needs to be trusted. The cases we see most often are:
Many people using Google Apps don’t have SPF configured. That’s our current understanding of this issue. If you have more insight on the issue, we’d love to update this post, so please add it in the comments below.