Why are emails from sendmail rejected by some mail servers?

Question

send mail

Hello!
Happy New Year, everyone! I ask for help on this issue: I installed the engine for Egea ’s blog on DigitalOcean according to the instructions from this blog “ How to start Egea on DigitalOcean ”, but I ran into such a problem that the engine does not send letters when posting comments from users on the blog, then I remembered that such a simple thing as can be responsible for the transport of letters sendmail, installed it, but did not configure anything. I left a test comment and received a letter to my inbox (Google Apps), but when I decided to reply to this comment, I could not send anything to Mail.ru sendmail. I looked in the file mail.logalong the path /var/log, and there I saw the following:

Dec 30 22:55:03 egeya-ubuntu-512mb-ams3 sendmail[29559]: gethostbyaddr(188.166.17.115) failed: 1
Dec 30 22:55:03 egeya-ubuntu-512mb-ams3 sendmail[29559]: gethostbyaddr(10.18.0.5) failed: 1
Dec 30 22:55:03 egeya-ubuntu-512mb-ams3 sendmail[29559]: alias database /etc/mail/aliases rebuilt by root
Dec 30 22:55:03 egeya-ubuntu-512mb-ams3 sendmail[29559]: /etc/mail/aliases: 0 aliases, longest 0 bytes, 0 bytes total
Dec 30 22:55:03 egeya-ubuntu-512mb-ams3 sm-mta[29666]: gethostbyaddr(188.166.17.115) failed: 1
Dec 30 22:55:03 egeya-ubuntu-512mb-ams3 sm-mta[29666]: gethostbyaddr(10.18.0.5) failed: 1
Dec 30 22:55:03 egeya-ubuntu-512mb-ams3 sm-mta[29668]: starting daemon (8.15.2): [email protected]:10:00
Dec 30 22:58:33 egeya-ubuntu-512mb-ams3 sendmail[29831]: uBUMwWfQ029831: from=www-data, size=709, class=0, nrcpts=1, msgid=<[email protected]$
Dec 30 22:58:33 egeya-ubuntu-512mb-ams3 sm-mta[29832]: uBUMwX0E029832: from=<[email protected]>, size=972, class=0, nrcpts=1, msgid=$
Dec 30 22:58:33 egeya-ubuntu-512mb-ams3 sendmail[29831]: uBUMwWfQ029831: [email protected], ctladdr=www-data (33/33), delay=00:00:01, xdelay=00:00:00, mailer=$
Dec 30 22:58:33 egeya-ubuntu-512mb-ams3 sm-mta[29834]: STARTTLS=client, relay=aspmx.l.google.com., version=TLSv1.2, verify=FAIL, cipher=ECDHE-RSA-AES128-GCM-SH$
Dec 30 22:58:33 egeya-ubuntu-512mb-ams3 sm-mta[29834]: uBUMwX0E029832: to=<[email protected]>, ctladdr=<[email protected]> (33/33), de$
Dec 30 23:05:28 egeya-ubuntu-512mb-ams3 sendmail[29867]: uBUN5Sp4029867: from=www-data, size=468, class=0, nrcpts=1, msgid=<[email protected]$
Dec 30 23:05:28 egeya-ubuntu-512mb-ams3 sm-mta[29868]: uBUN5S9T029868: from=<[email protected]>, size=731, class=0, nrcpts=1, msgid=$
Dec 30 23:05:28 egeya-ubuntu-512mb-ams3 sendmail[29867]: uBUN5Sp4029867: [email protected], ctladdr=www-data (33/33), delay=00:00:00, xdelay=00:00:00, mailer=r$
Dec 30 23:05:28 egeya-ubuntu-512mb-ams3 sm-mta[29870]: STARTTLS=client, relay=mxs.mail.ru., version=TLSv1.2, verify=FAIL, cipher=AES256-GCM-SHA384, bits=256/256
Dec 30 23:05:29 egeya-ubuntu-512mb-ams3 sm-mta[29870]: uBUN5S9T029868: to=<[email protected]>, ctladdr=<[email protected]> (33/33), del$
Dec 30 23:05:29 egeya-ubuntu-512mb-ams3 sm-mta[29870]: uBUN5S9T029868: uBUN5T9T029870: DSN: Service unavailable
Dec 30 23:05:30 egeya-ubuntu-512mb-ams3 sm-mta[29870]: uBUN5T9T029870: to=<[email protected]>, delay=00:00:01, xdelay=00:00:01, mail$
Dec 30 23:17:19 egeya-ubuntu-512mb-ams3 sm-mta[1627]: gethostbyaddr(188.166.17.115) failed: 1
Dec 30 23:17:19 egeya-ubuntu-512mb-ams3 sm-mta[1627]: gethostbyaddr(10.18.0.5) failed: 1
Dec 30 23:17:19 egeya-ubuntu-512mb-ams3 sm-mta[1634]: starting daemon (8.15.2): [email protected]:10:00

As I understand it, the letter went through to the pisma.ws domain, but on the Mail.ru side, the letter did not pass verification and was rejected by their server.
I have little knowledge in these matters, so maybe someone from the experts in this field can advise me some solution for my problem, or maybe in the domain settings for my blog, you can add some DNS records so that the letters do not got into spam and was not rejected by mail servers.
* * *
UPD 1 : After a little googling, I managed to find such a note " Mail settings reminder ". A person uses the Egea engine, but, unfortunately, it is not clear to me how to do everything that he describes in his material. Maybe someone can advise?