Some excellent resources if you’re hosting your own domain with Office 365. When doing so you’ll need to create a TXT SPF record for email. The SPF Generator Framework Wizard below was very helpful generating an SPF record that accounted for my webhost generated email (web forms) and email on 365 servers –
v=spf1 ip4:xx.xx.xx.xx include:spf.protection.outlook.com -all
Office 365 DNS – Details of External DNS records for Office 365 including SPF
https://support.office.com/en-us/article/External-Domain-Name-System-records-for-Office-365-c0531a6f-9e25-4f2d-ad0e-a70bfef09ac0
How Microsoft 365 – uses Sender Policy Framework (SPF) to prevent spoofing
https://docs.microsoft.com/en-us/microsoft-365/security/office-365-security/how-office-365-uses-spf-to-prevent-spoofing?view=o365-worldwide
Validate SPF – with these excellent SPF Record Testing Tools
http://www.kitterman.com/spf/validate.html
IP Address – How to Designate an IP Address as Permitted Sender
http://www.x-pose.org/2013/10/how-to-designate-an-ip-address-as-permitted-sender/
Example DNS settings – SPF TXT record with IPV4 for my webhost and Outlook.com for user email: v=spf1 ip4:xx.xx.xx.xx include:spf.protection.outlook.com -all
