Our data indicates Showpad supports full DMARC compliance via SPF and DKIM. See this article: https://help.showpad.com/hc/en-us/articles/211957689-Setting-up-Sending-domains-for-Showpad
To configure DKIM, see: https://help.showpad.com/hc/en-us/articles/211957689-Setting-up-Sending-domains-for-Showpad
To configure SPF, see: https://help.showpad.com/hc/en-us/articles/211957689-Setting-up-Sending-domains-for-Showpad