Sonic.net

@charliefenton the DNS system internally operates on fully qualified names only: _dmarc.cfenton.com.

The omission of the domain name and trailing period is a convenience for user interfaces and DNS server configuration files.
@joemuller is correct the "@" is another convenience feature which represents the domain name and trailing period.

@JoeMuller: Mailchimp has responded that the _dmarc record you added is acceptable, and my testing confirms that it does what is needed. Thank you for your help.