What causes email to go to the spam folder?

A quick guide to some of the things ISPs look for to decide if it should go to the Inbox or the spam folder.

Recently a former colleague reached out to me on Linkedin to ask:

I have a question regarding email delivery. What cause emails to go into someone’s spam email box? I understand that there maybe(sic) filters that looks at the content to make that determination. I would think there are many other factors.

I replied:

Yes, there’s quite a number of things that can cause mail to go to the spam folder. The contents of the message are a big factor. Of course every ISP applies different rules, so what causes mail to go into the spam folder of a Yahoo! mailbox will differ from what matches the rules on Gmail, or Hotmail, etc. Some ISPs will allow certain mail through, but put it in the Spam folder that other ISPs would just reject outright when the sending mail server connects to send it.

Are you having a specific problem that you’re trying to solve?

He responded:

I don’t have a specific problem. Just interested in understanding how spam filtering works. Since I know an expert, why not ask directly.

Are there headers the ISP look at to validate the email?

I wrote up a quick primer on some of the esoterica of spam filtering.
This is by no means comprehensive, and not guaranteed 100% accurate.

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