How to write an SMTP client so as not to get on the list of spammers?

Question

Email

Wrote a client based on the SMTP protocol. After sending from my computer to my own email, I got into the list of spammers. Mylru swears by an error:

550 spam message rejected. Please visit help.mail.ru/notspam-support/id?c=...

How to send emails to YOURSELF and not be a spammer?

Answer 1

Yes, there is an evil spam detector there ...
Write a message using a real mailer and repeat as many fields as possible from this mailer.
Perhaps the most important is the User-Agent. 80 percent of bot filters refer to it.

Answer 2

This article has some basic guidelines.

Answer 3

Both letters are sent from their smpt server.
Everything is simple here. If the letter is not signed with DKIM, it is 90% spam. Since all mailers will mark it as suspicious.
Encryption: yes
If the email is signed with DKIM, it is verified.
Encryption: yes