My server sends the email to GMAIL.COM. If I try to send an email to my Yahoo mail address or if I send an email to an address with .ru extension it will be never received by the recipient.
Some messages: /var/log/maillog
Feb 9 18:42:13 myserver postfix/smtp: B6D5D530091: to=<firstname.lastname@example.org>, relay=f.mx.mail.yahoo.com[18.104.22.168]:25, delay=3.1, delays=0.04/0/2.5/0.55, dsn=4.0.0, status=deferred (host f.mx.mail.yahoo.com[22.214.171.124] said: 451 Message temporarily deferred -  (in reply to end of DATA command))
Feb 9 18:50:09 myserver postfix/smtp: 5C4FA530093: to=<email@example.com>, relay=a.mx.mail.yahoo.com[126.96.36.199]:25, delay=3.7, delays=0.04/0.01/1.4/2.2, dsn=2.0.0, status=sent (250 ok dirdel)
Feb 9 19:42:22 myserver postfix/smtp: C774E530097: host d.mx.mail.yahoo.com[188.8.131.52] refused to talk to me: 421 Message from (184.108.40.206) temporarily deferred - 4.16.50. Please refer to http://help.yahoo.com/help/us/mail/defer/defer-06.html