I have DMARC DNS records for my personal domain, and can verify that Fastmail alias targeting of a Gmail account (with SRS enabled) passes SPF and DKIM. Here are the munged received headers at my Gmail account:
Code:
Received-SPF:
pass (google.com: domain of SRS0=sBhS=WJ=mydomain.com=bill@srs.messagingengine.com designates 66.111.4.26 as permitted sender) client-ip=66.111.4.26;
Authentication-Results: mx.google.com;
spf=pass (google.com: domain of SRS0=sBhS=WJ=mydomain.com=bill@srs.messagingengine.com designates 66.111.4.26 as permitted sender) smtp.mail=SRS0=sBhS=WJ=mydomain.com=bill@srs.messagingengine.com;
dkim=pass header.i=@mydomain.com;
dmarc=pass (p=NONE dis=NONE) header.from=mydomain.com
My guess is still that something else (spam markers, sending address or domain, or some IP address) is causing Gmail to consider this as spam.