In order to allow emails to be sent from your domain, we must set up an SPF record mechanism giving Mandrill (the emailing service we use) permission to send emails under your domain name. A domain can only contain up to 10 mechanisms in its SPF record; exceeding this limit means that there will not be enough room to add the required Mandrill mechanism.
There are 3 options when it comes to resolving this issue:
The most straightforward option is to delete some of the pre-existing SPF mechanisms within the SPF record (such that there are 9 or less). The SPF txt record can be found within your domain's DNS records. https://www.dmarcian.com/spf-survey/ is a tool that can be used to count and visualize SPF mechanisms.
It is important to note that some of the mechanisms might be important to pre-existing emailing systems. You should contact your Email Administrator or Support for assistance.
Another option is to create a new subdomain and using that subdomain to send emails instead. The SPF record on this domain will be empty and thus not exceed the 10 mechanism limit. This method requires more technical know-how and should be done by the Email Administrator or Support.
Finally another option is to modify your SPF records through a method known as 'SPF Flattening', which reduces the number of SPF mechanisms. Information regarding this can be found at:
https://success.salesforce.com/ideaView?id=0873A000000cMUpQAM. https://www.dmarcian.com/spf-survey/ has a 'SPF Record Flattener' at the bottom of the page showing examples of this.
Once you have a domain with an acceptable SPF record, you can submit it in 'Step 1' and proceed to 'Step 2' of the emailing process.
SPF/DNS changes can take up to 24 hours to apply (depending on the provider).