sudo php -r "mail('dj_alximik@mail.ru', 'Тестовое письмо с вашего сайта', 'Привет. Это тестовое сообщение.');"

127.0.0.1   localhost.localdomain localhost
127.0.1.1   RPC.domain RPC

# The following lines are desirable for IPv6 capable hosts
::1     ip6-localhost ip6-loopback
fe00::0 ip6-localnet
ff00::0 ip6-mcastprefix
ff02::1 ip6-allnodes
ff02::2 ip6-allrouters
127.0.0.1       test.jkl
127.0.0.1       lessonsPHP7
127.0.0.1       calc
127.0.0.1       mytest.loc

Nov 30 08:13:33 RPC sm-mta[4942]: wATKMUrX015802: to=<djalximik@gmail.com>, ctladdr=<www-data@RPC.domain> (33/33), delay=09:51:03, xdelay=00:10:54, mailer=esmtp, pri=210377, relay=alt4.gmail-smtp-in.l.google.com. [74.125.129.26], dsn=4.0.0, stat=Deferred: Connection timed out with alt4.gmail-smtp-in.l.google.com.
Nov 30 08:13:33 RPC sm-mta[4942]: wATKMUrX015802: wAU62dWZ004942: sender notify: Warning: could not send message for past 4 hours
Nov 30 08:13:33 RPC sm-mta[4942]: wAU62dWZ004942: to=<www-data@RPC.domain>, delay=00:00:00, xdelay=00:00:00, mailer=local, pri=30000, dsn=2.0.0, stat=Sent
Nov 30 08:13:33 RPC sm-mta[4942]: wATKI8XB015540: to=<djalximik@gmail.com>, ctladdr=<www-data@RPC.domain> (33/33), delay=09:55:25, xdelay=00:00:00, mailer=esmtp, pri=390377, relay=alt4.gmail-smtp-in.l.google.com., dsn=4.0.0, stat=Deferred: Connection timed out with alt4.gmail-smtp-in.l.google.com.

Trying 127.0.0.1...
Connected to localhost.localdomain.
Escape character is '^]'.
220 RPC.domain ESMTP Sendmail 8.15.2/8.15.2/Debian-3; Fri, 30 Nov 2018 08:20:02 +0200; (No UCE/UBE) logging access from: localhost.localdomain(OK)-localhost.localdomain [127.0.0.1]