Why are messages moved to the spam/junk folder (receiving locally, not sent to an external server)?

Question

Why are messages moved to the spam/junk folder (receiving locally, not sent to an external server)?

Answer

There are a few potential causes for messages being placed in the spam/junk folder. Apache SpamAssassin is the most common reason. User-created email filters, email clients, or anti-virus software on the user's computer can also move messages into the spam/junk folder.

Additional resources

How to find SpamAssassin scan results

How can I check why SpamAssassin applied a particular score?

How to find email headers

Spam Filters: Calculated Spam Score Settings

Email Filters

How to enable the Dovecot mail_log plugin