a letter came about that allegedly someone hacked me there and now demand loot.
The content of the letter is not particularly interested in me, but just how he sent a letter to the same mail from my post site? Where to look for a hole on the server? (on other domains there is no such garbage)

Answer 1, Authority 100%

came a similar letter, do not worry at all, this is a standard e-mail newsletter, another spammer and yes, SMTP allows you to write in “from” anyone …

Answer 2, Authority 100%

How did he send a letter to the same mail from my post?

Install on the other machine Package SWAKS (Sudo Apt Install Swaks ) and you can repeat the trick:

swaks --body "you broke" --to [email protected] --From [email protected] - Add-Header "Subject: Hi with \" Server \ ""

Where to look for a hole on the server?

Most likely it is not. If you have a digital signature of DKIM, then the fee will not be checked. Yandex has a “red lock”:

You can independently check through the dkimverify utility.

Also look at the headers: from , among them there will be an IP attacker.