Adding the _spf.mxguard.uk include without breaking SPF

When you enable the smarthost you must authorise MxGuard's sending IPs in your SPF record, or receivers will fail SPF on your relayed mail.

What to add. Insert include:_spf.mxguard.uk into your existing SPF TXT record — don't create a second SPF record (a domain must have only one).

Example. If you currently publish:

v=spf1 mx ~all

change it to:

v=spf1 mx include:_spf.mxguard.uk ~all

Keep the all mechanism at the end.

Watch the 10-lookup limit. SPF allows a maximum of ten DNS-lookup mechanisms (include, a, mx, etc.); exceed it and SPF returns permerror, which is worse than not having the include at all. If your record is already crowded with includes for other senders, tidy up unused ones when you add MxGuard's. After publishing, send a test message and confirm SPF passes at the receiving end.