ext.imap1a.mail.m.oml.ru has no CNAME records.
Question and response
Question
dig ext.imap1a.mail.m.oml.ru. NS
Answer
The response has no answer records.
Authority
oml.ru. 700 SOA ns.megagroup.ru. dnsmaster.megagroup.ru. 1139964 21600 1800 1209600 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0