ext.imap1b.mail.m.OML.RU has no CNAME records.
Question and response
Question
dig ext.imap1b.mail.m.OML.RU. NS
Answer
The response has no answer records.
Authority
OML.RU. 524 SOA ns.megagroup.RU. dnsmaster.megagroup.RU. 1139964 21600 1800 1209600 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0