Google says it's my fault; I say Google has its collective head up its ass in marking a valid email as spam. Their official word is "Don't act like the bad guys" and you won't have problems. Never mind that it's 100% RFC-compliant, they say it's bad guy behaviour.
I agree. Sending mail to google (and also hotmail/outlook) has become very unreliable. E.g. when using @amsat.org or other forwarding services like @veron.nl or @vrza.nl (Dutch amateur radio societies) it will usually not work.
I do not really want to use my ISP mail address, so it looks like I have to register my own domain to get this fixed. For now, I have set an autoreply (from the ISP mail address) encouraging people to use a different address and look in their spambox when they expect a reply.
I hear that it is customary in the circles of gmail/hotmail/outlook users to send a Whatsapp message "I have sent you an e-mail"...
Rob