sendmail send mail too slow

After setup sendmail in my instances running in Digital Ocean cloud, I tried to send few mails using simple php function mail().

And unfortunately, mails sends too slow. About 60 seconds needed to send one email.

i go to logs (/var/log/mail.err), to see whats happens:

Sep 17 11:40:01 ibragimovby sm-msp-queue[6689]: My unqualified host name (ibragimovby) unknown; sleeping for retry
Sep 17 11:41:01 ibragimovby sm-msp-queue[6689]: unable to qualify my own domain name (ibragimovby) -- using short name

So, after i add following line in /etc/hosts:

127.0.0.1       localhost localhost.localdomain ibragimovby

and emails started sending immediately.