To authorize MailPlus to send emails on your behalf you will have to include them to your SPF record. The SPF mechanism used by MailPlus is shown below.
MailPlus also support DKIM and can be configured to DKIM sign your emails for further resilience and failover if SPF fails. Please contact your MailPlus account administrator or support person for instructions on how to set up DKIM.
Create a free OnDMARC account to test your configuration.