I use email hosting from my domain name provider (email in my profile). I regularly find out that personal emails from me end up in people's spam folders on gmail. As far as I can guess from digging into email headers, gmail is upset because my domain name provider's email server doesn't use whatever the latest mail-server signing mechanism gmail expects is. There's absolutely nothing I can do about that, and no obvious way I can get gmail to stop treating my mail as questionable and throwing it in people's spam folders. And it's not at all obvious how to debug this.

I get why gmail isn't transparent about all their spam-filtering mechanisms. But when personal email to gmail users is being regularly classified as spam, that produces a serious usability problem for non-gmail users.