[kureikain]

Yes, I heard you.

To gmail point of view, they don't care about forwarding or replying to an email someone send you at all. All they care seems is about the message follow standard best practice(SPF/DKIM/DMARC/PTR) and send from a good IP. Sometime your IP can be flagged as spam wrongly on DNSBL. But it's very easy to get remove from DNSBL. Just submit a removal request, explain your situation and it's usually remove within a few days.

Also, the reason while your important emails are marked as spam is depended on gmail mx server you connected to. I don't know the detail but I have seen gmail mx server rejected/spammed my email, but on subsequent try if I connect to different one(by resolving other MX record), the email went through just fine.

If you send me an email [e-mail] I can try to spot if anything may randomly flagged your email.

Also, did you have multiple SMTP servers?