As part of the product we offer we send emails on behalf of our clients. Because the emails don't come from an IP associated with the client they are sometimes flagged as spam.

We advised some of our clients to add an SPF record approving us to send emails on their behalf. We saw immediate improvement in deliverability rates after making the change however one of our clients was notified by his hosting provider that the SPF record we suggested to add would "slightly restrict" all emails that don't come from our servers (including our client's own servers).

The record we use is this:

v=spf1 a mx include:ourdomain.com ~all

So my question is if the warning we received about this is correct and if so why and what can be done to solve this (allow sending email both from original domain and by ourselves).