If you have emails rejected when sending using a custom domain with error messages like:

• 550-5.7.26 This mail has been blocked because the sender is unauthenticated
• 550-5.7.26 Gmail requires all senders to authenticate with either SPF or DKIM.
• 550 SPF error
• 550 Blocked

or similar error, you will need to update or add an SPF record in your Domain DNS entries as per below:

Record Type: TXT

Value: v=spf1 include:spf.eu-west.atmailcloud.com -all

It should be noted this type of setup is NOT required for standard consumer customers. Please contact the IT service/provider who setup your custom domain for support should it be required.