I had a few notification emails from this forum wind up in my spam folder, and it looks like there’s an SPF failure happening. Seem like the server’s IPv6 address isn’t in the SPF record:
$ drill TXT forum.k9mail.app forum.k9mail.app. 28800 IN TXT "v=spf1 a mx ip4:220.127.116.11 -all"
Received-SPF: Fail (mailfrom) identity=mailfrom; client-ip=2a05:bec0:1:17:9005:afff:fe05:8c85 helo=forum.k9mail.app; firstname.lastname@example.org; receiver=<UNKNOWN> Received: from forum.k9mail.app (unknown [IPv6:2a05:bec0:1:17:9005:afff:fe05:8c85]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits)