To allow CRM-service to send out email from a domain, the SPF record hosting the email requires a few settings. You can either insert the following MX record to the SPF record:
or you can add the following value to the SPF record (recommended):
Why give a permission to CRM-service to send out e-mail?
The Sender Policy Framework (SPF) is a simple email validation system designed to detect email spoofing by providing a mechanism to allow receiving mail exchangers to check that incoming mail from a domain comes from a host authorized by that domain’s administrators. The list of authorized sending hosts for a domain is published in the Domain Name System (DNS) records for that domain in the form of a specially formatted TXT record. Publishing and checking SPF records can be considered as anti-spam techniques.
CRM-service is a domain that is authorized to send email on your behalf and that is why MX record should have an SPF record specifying the policy if it is used either in an email address.
This is used in the CRM and in Email Contact Portal (ECP) to authorize the sending email addresses (see the picture below).