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Trying to send email message using python's smtplib via gmail:

import smtplib

msg = "rn".join([
 "From: " + email_host,
 "To: " + email_recipient,
 "Subject: subject",
 "",
 email_msg
])
server = smtplib.SMTP('smtp.gmail.com:587')
server.ehlo()
server.starttls()
server.login(email_host, email_pwd)
server.sendmail(email_host, email_recipient, msg)
server.quit()

For popular services such as yandex, yahoo and gmail it works perfect. But it's impossible to send a message to domain-based emails (login@some_domain.com).

What's the problem?

Email services apply restrictions since there's no authentication when you're sending those emails.

E.g.
https://support.asperasoft.com/hc/en-us/articles/216128488-How-to-use-Gmail-as-an-SMTP-relay-for-notification-testing

This SMTP server does not require TLS or authentication. It is
restricted because emails can only be sent to other Gmail addresses or
Google apps users--therefore it may only make sense for testing within
an internal network that uses Gmail or Google apps. Limits apply per
recipient.

I.e. these services don't mind sending emails within their own domain, but don't want malicious users sending to other domains.

Well, searching for the answer, I found out that Google Mail service doesn't allow to send messages to domain-based emails via 587 SMTP port, because it requires to use TLS.

I decided to switch onto 465 SMTP port as said in this post, so it works now perfect! Also, for testing purposes, I used another Mail Service (Yandex), which supports only 465 SMTP port - it works as well.