Yes. It’s happened to me and it is a head fuck. The email was from a business with a perfectly legit email address.

You haven’t mentioned what email provider they’re using. That matters a lot. Not all providers have smart filter. No need to attribute to malice what can be explained by reason.

Yes, and even more chance of that happening today than 5 years ago. Reason: because of the modern day prevalence of the ‘fake reply’ SPAM and Phishing emails. Spammers and phishers are now drafting fresh messages mocked up to look like replies in existing email threads…older spam detection used to let these types of messages slip through because they thought they must be legitimate replies, and so naturally spammers started exploiting that to slip past detection. Modern detection no longer gives apparant replies a free pass.

Absolutely. Ask Apple. Even the “VIP” all end up in the spam folder all the time too, and there’s no way to tell it “this isn’t spam”.

In my experience, yes. When I finally got the official job offer from a HR person I’d previously communicated with, the email landed in spam.

I had an email go to spam in the middle of a discussion with my insurance company. Even though I had already had 5 exchanges with them.

Yes. Email basically makes no sense whatsoever, it’s a rotten pile of hacks on hacks on hacks that’s been around for decades. Anything can end up anywhere.

It shouldn’t, but it can, had it happen a bare handful of times but it’s really rare.

To expand on what others have said, once you send the email, one of two things might happen:

1.) The receiving email server might report the email as delivered to the recipient, or

2.) Your email might get “bounced”, where the recipient is not notified of the email and the email is returned to you with a notice stating why the email was not delivered.

The second one is what happens if the email server is “sure” your email is spam. But even if the email is marked as delivered, that doesn’t mean it goes in the inbox. Secondary checks determine which box it goes in, and that box might be “Spam”. This secondary process is entirely internal to the mall provider, where the initial checks before delivery usually rely on a SpamAssassin instance.

Both are influenced by a number of factors, but the biggest are DNS (Domain Name Service) records on your domain which show you’re a legitimate sender. These are:

• SPF: Sender Policy Framework
• DKIM: Domain Keys-Identified Mail
• DMARC: Domain-based Messaging, Authentication, Reporting, and Conformance

A comprehensive explanation and guide can be found here: https://www.dmarcly.com/blog/how-to-implement-dmarc-dkim-spf-to-stop-email-spoofing-phishing-the-definitive-guide

But the short of it is that if your SPF or DKIM are wrong or missing, your emails are much more likely to be rejected or to land in spam. You won’t lose points (yet) for not having DMARC, but if you have SPF and DKIM set up correctly it gives about 10% better chance of being delivered (but it may not affect whether you get to the inbox, depending on those secondary checks).

You can use https://mail-tester.com to check for these and other issues that would stop your emails getting to the inbox. Don’t worry about “reverse DNS” not matching, or if it says you don’t have an “unsubscribe header”. RDNS can’t be expected to match anymore and I don’t think it’s a good spam indicator. Unsubscribe headers are only for mailing list emails, so if you’re not testing a marketing email from somewhere like MailChimp then you won’t have an unsubscribe header. Finally, being on SORBS is not the end of the world, and blacklists generally can be ignored unless your email provider is small, or if you run your own server.

Finally, don’t ever think “it’s been working fine so it shouldn’t stop working now”. SPF and DKIM weren’t necessary five-ish years ago, now they’re mandatory; DMARC itself hasn’t been taken very seriously to date but Microsoft just recently announced they’re actually going to start paying attention to it. On top of standards changing, unless you own and run the email server in your dwelling, you don’t have the control necessary to say “nothing has changed”. Microsoft can and does change their system constantly behind the scenes: applying patches, updates, retiring old servers and configuring new ones into the cluster, and so on. What was is not relevant; you can only look at what is, and fix that.

Yes, but the mail provider/software you’re using and a bunch of other variables do play the role in this equation.

A suggestion: to confirm it, have a test conversation with somoene and mark the last message as SPAM, to see how your environment is going to react.

Microsoft Outlook will file your mail away as spam for a while if it doesn’t like the color of the shirt you’re wearing. It’s arbitrary and seemingly random, even if you get SPF/DKIM/ARC/DANE/reverse PTR right. Google too, though it seems to be less random in what it does or doesn’t accept.

There’s nothing technically preventing any email from being marked as spam. You can turn on the option to ask for delivery receipts if you want confirmation that the email has been delivered, but they won’t be sent automatically (user has to click a button to confirm).

Absolutely yes. Even if you have marked all their previous emails as non-spam manually.

Email needs to change. It’s incredibly insecure and the volume of spam/phishing/scams is overwhelming. Most people will not notice this, but if you open a couple of domains or work at a company that has secrets worth getting at, you’ll notice how bad it is.