I just enabled DMARC reporting and am looking through my first batch of entries (actually, just one entry). One thing caught my eye -- an SPF failure due to mismatched sender IP address (126.96.36.199), which resolves to imap1.us.opalstack.com.
I'm not sure why this IP would be sending an email from my domain. I did personally only send one email during that period, was that it? I send through gmail authenticated to smtp.us.opalstack.com. My SPF record checks out I think:
v=spf1 include:spf.opalstack.com include:_spf.google.com -all
That IMAP IP address is not in the spf.opalstack.com set of values, FWIW.