The gsys.biz domain has had an SPF record in place for several years. Previously its MX records were set-up to rely on email forwarding by the domain registrar. Unfortunately the registrar in use does not support SRS so all emails received are marked as failing SPF checks.
Recently the number of "bounce" email being received has been getting ridiculous. In an effort to combat this the SPF record should be changed from a soft to hard fail, DKIM signing introduced for outgoing mail and a DMARC DNS entry generated. Unfortunately, the use of the registrars email forwarder prevents a problem. The solution? Use the email server bundled with our web hosting package.
In the first place the MX record was set to point to mail.gridhost.co.uk. All appeared well, but over the course of a few weeks the occasional email was going missing.
Raising this with Vidahost, it appears the MX record was setup wrongly. It should have had an entry of mail3.eqx.gridhost.co.uk.