The SPF include and optional DKIM signing for outbound

Two authentication details make the difference between your relayed mail landing in the inbox or the spam folder.

SPF include (do this). When you route outbound through MxGuard, our IPs are now sending on your behalf, so your SPF record must authorise them. Add include:_spf.mxguard.uk to your domain's SPF record. Without it, receivers see mail 'from' your domain arriving from our IPs and SPF fails.

DKIM signing (optional). MxGuard can DKIM-sign your outbound mail using a provided selector. Most customers already sign at their own mail server before the message reaches the relay, in which case you can leave MxGuard's signing off — the existing signature travels through untouched. Turn it on if your server doesn't sign and you want the relay to add DKIM for you.

Between a correct SPF include and a valid DKIM signature (from either end), your relayed mail authenticates cleanly at Gmail, Microsoft and Yahoo — which is the whole point of using a reputable smarthost.