Why do messages I send end up in recipients’ “spam folders”?
If you’re seeing this problem, the first thing to keep in mind is that each receiving mail company has its own independent spam rules. They don’t share the rules with senders or us, and we can’t control their rules, of course.
You should try to find if it’s happening just at one company (such as Gmail), or at multiple companies. Then try to get one of the customers of those companies to ask their mail support people why your message is being put in the spam folder. If you can do that, you’ll know what to focus on and can probably solve it.
If you can’t get an answer, you’ll have to try to figure out the cause yourself. The main thing that ISPs have in common is that they’ll put mail in the spam folder for all their customers if some of their customers mark it as spam or unwanted mail. It doesn’t matter whether you (or we) consider it spam or not: What matters is whether recipients are marking it that way. You need to make sure that doesn’t happen.
If you send any kind of bulk mail, this page shows our guidelines for avoiding this problem. It’s pretty strict, but it’s what receiving mail companies tell us is necessary to prevent your domain name from getting a poor reputation and being treated as spam.