To ensure your emails are successfully delivered, and limit the chances of being flagged as spam, you will need to add a record to your SPF that includes the list of email addresses given to Marketo in this welcome email.
Creating SPF record for Marketo
Login to your DNS management console
Navigate to Tools & Settings > DNS Template
You need to edit the DNS TXT record related to SPF
An SPF TXT record for Marketo will have the following syntax:
yourdomain.com TXT v=spf1 include:mktomail.com -all
Publish this record in your DNS records section for your respective domain.
Note: If you don’t have an SPF record previously for your domain, simply publish the above-mentioned record to enable SPF for outgoing emails using Marketo. If you do have an existing SPF record in your DNS, you need to modify it to include Marketo, as shown below:
Previous SPF record: v=spf1 include:spf.zoho.com -all
Modified SPF record: v=spf1 include:spf.zoho.com include:mktomail.com -all
Wait for 24-48 hours to allow your DNS to process the changes