Will adding a second SPF record mess up my DNS, or will it be like adding an extra nameserver?
(i.e. it only helps, not hurts)
From RFC 4408:
I'm not entirely sure what you want to achieve by adding a second record, but if it is something like adding extra hosts/networks as valid/invalid senders, you can probably do everything you want to with just the one record - just add whatever you wanted to the end of the line.
An SPF record can include multiple domains (multiple mechanisms is the correct term), so more than one spf record will never be needed.
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