When using Marketo it is not a requirement to set up DNS text records for SPF and DKIM. However, Marketo recommends setting up SPF and DKIM because it improves the deliverability of your mailings. Configuring and implementing one or both of these records is a way to verify that the server sending your mail is authorized to do so.
If a recipient domain is configured to check for SPF and/or DKIM and those DNS records are available and your mail passes the SPF/DKIM check, it further reinforces its good reputation. Not implementing SPF/DKIM records does not add to or subtract from its reputation, it’s just not there.
Please note, not all domains check for SPF/DKIM and if this is the case, again, the presence of these records does not add or subtract from your mail’s deliverability.
There is no negative effect to setting up these records, and it can improve your deliverability. It is for these reasons that Marketo recommends setting up these DNS records and configuring their use in your instance of Marketo.
For more information on how to set up and configure SPF/DKIM, please read our KB article here.