Why is sending email not working?

Question

Exim

vesta / centos 7 / exim
when sending emails from the round cube, the following SMTP error occurs (550): Unable to set sender "[email protected]" (Access denied - Invalid HELO name (See RFC2821
4.1.3))
a problem with the server (it is not possible to fix it) due to the incorrect operation of the dns server of the hoster (the hoster went out)
And the record - on the ip of the server from the registrar is configured - 93.170
. also at the registrar / hoster automatically created
ptr registered at the hoster on the server - 149.123.170.93.in-addr.arpa IN PTR m-cppgroup.ru
MX 10 mail.m-cppgroup.ru.
Also, when creating a server, the default setting is to use hosting dns.
Advise where to look and how to fix it?

Answer 1

Invalid HELO name
here and look - in Google and read options
1) check config
2) check dns reverse
I think 1 is enough