Mail of "normal" contacts in spam folder

After the latest update a lot of mails from people that send me a mail for the first time are stored in the Spam folder.How can I prevent this?

can you send me an example of the “normal” email you’re referring to?
Send the email as an attachment in .eml file to my email [email protected]

Also are you using any rules for sorting out emails?
What mail service are you using?

Thank you,