One is capable of sending DMARC compliant email because they support custom SPF and DKIM configurations that achieve DMARC alignment. One is a Hosting Platform and is often used by Application Development and Operations teams.
We recommend opening a support ticket to get help with custom DKIM configurations as we couldn’t find any relevant ‘how to’ documentation on their site. The most recent support alias we found was at https://help.one.com/hc/en-us/requests/new
The link just below is a resource on how to configure this source to send SPF-aligned email on your behalf. You will complete this configuration by (1) following the source's instructions and (2) adding the required entries into your DNS. https://help.one.com/hc/en-us/articles/115005595945-Add-an-SPF-record