We used yandex mail provider for our email service. We faced this problem (gmail couldn’t verify that this message was sent by and not a spammer) for our email address too.
You can solve this problem by adding a TXT record to your DNS Settings.
Add the below TXT record and wait for few minutes, all outgoing emails will have digital signature and thus we will not see any such problem.
DNS SETTINGS TXT RECORD Value: v=spf1 redirect=_spf.yandex.net HostName: @