I've done some digging, and you can use the following SPF entry for true "set it and forget it" SPF protection:
"v=spf1 ip4:1.2.3.4 include:smtproutes.com include:smtpout.com ~all"
...where "ip4:1.2.3.4 " is the IP address of your mail server. If you have other sending concerns to account for and have a more elaborate SPF record to maintain, the special sauce needed to cover MaxMail is the portion marked:
"include:smtproutes.com include:smtpout.com"Add that portion to your SPF and worry no more about MailProtection and how it relates to your outbound mail's SPF.