I'm having the opposite problem. I have a custom domain email address through a webhost, which gets forwarded to Gmail. I've been using Gmail as my client since day 1 (around whenever Gmail first came out). Some time in early summer I stopped getting emails from my wife, who uses a plain Gmail address. I was missing messages in threads if she was the sender. This has never happened before. So I checked the logs on the webhost, and it turns out Gmail was rejecting mail sent from her with an error that her address is spam. And I don't mean it went into the spam folder. It straight up rejected the messages and wouldn't go to the Gmail servers at all. I found this crazy, because it was rejecting a Gmail address that has been used for years. And in message threads that included others with Gmail addresses. And if she emailed me to my actual Gmail address, it was fine and not spam. But her address (and only hers) gets marked as spam if sent to my domain.

So my solution was to start using Thunderbird. Her messages were never being rejected by my domain and all the threads are there intact.

Edit: This was the SMTP error that Google was telling my domain: "Our system has detected that this message is likely unsolicited mail. To reduce the amount of spam sent to Gmail, this message has been blocked."