Why do emails from the site end up in spam?

Question

Plesk

Good afternoon!
When sending a letter from the site's mail ([email protected]), the letters go to the addressee in spam.
I'm not talking about mailings, but about sending a regular letter.
Tested the site on .mail-tester.com . Result 10/10
Help me understand what is wrong.
I use vds server + Plesk
PS: if you need any additional information, please write and I will indicate it.

Answer 1

because it is impossible to describe antispam
in two sentences this way, but check at least the IP in the blacklists

Answer 2

Because the receiving mail server has antispam configured this way :) Criteria can be very different - up to your personal blocking :)

Answer 3

There are similar and beautiful effects here