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Why are CNAME not valid for sendmail email from sendmail?

I have domain "blabla.com" which is cnamed to "bla.bla.bla.uk." I sendmail it automatically picking up the correct domain name I want to send email from but once it hits a mail server like google the FQDN displayed is the A type record and not the CNAME.

Has anyone come across this before? Thanks for you help in advance.

SU@sbank (nineveh-r2011_1.5.3) 156 > nslookup bla.bla.bla.uk 8.8.8.8
Server:         8.8.8.8
Address:        8.8.8.8#53

Non-authoritative answer:

bla.bla.bla.uk       canonical name = blabla.com.
Name:   blabla.com
Address: 217.112.157.7X


SU@ban (nineveh-r2011_1.5.3) 173 > sendmail -v user@gmail.com
test5

.

useru@gmail.com... Connecting to [127.0.0.1] via relay...
220 lispac.lsbu.ac.uk ESMTP Sendmail 8.13.8/8.12.11; Mon, 4 Aug 2014 09:10:42 +0100
>>> EHLO bla.bla.bla.uk
250-bla.bla.bla.uk Hello lispac.lsbu.ac.uk [127.0.0.1], pleased to meet you
250-ENHANCEDSTATUSCODES
250-PIPELINING
250-EXPN
250-VERB
250-8BITMIME
250-SIZE
250-DSN
250-ETRN
250-DELIVERBY
250 HELP
>>> VERB
250 2.0.0 Verbose mode
>>> MAIL From:<iii@bla.bla.bla.uk> SIZE=6
250 2.1.0 <iii@lbla.bla.bla.uk>... Sender ok
>>> RCPT To:<user@gmail.com>
>>> DATA
250 2.1.5 <user@gmail.com>... Recipient ok
354 Enter mail, end with "." on a line by itself
>>> .
050 <user@gmail.com>... Connecting to gmail-smtp-in.l.google.com. via esmtp...
050 220 ***********************************************
050 >>> EHLO bla.bla.bla.uk
050 250-mx.google.com at your service, [217.112.157.X]
050 250-SIZE 35882577
050 250-8BITMIME
050 250-XXXXXXXA
050 250-ENHANCEDSTATUSCODES
050 250 XXXXXXXB
050 >>> MAIL From:<iii@bla.com> SIZE=300
050 250 2.1.0 OK fo6si32099947wjc.83 - gsmtp
050 >>> RCPT To:<user@gmail.com>
050 250 2.1.5 OK fo6si32099947wjc.83 - gsmtp
050 >>> DATA
050 354  Go ahead fo6si32099947wjc.83 - gsmtp
050 >>> .
050 250 2.0.0 OK 1407139864 fo6si32099947wjc.83 - gsmtp
050 <user@gmail.com>... Sent (OK 1407139864 fo6si32099947wjc.83 - gsmtp)
250 2.0.0 s748AgvS017005 Message accepted for delivery
alexandru.tautu@gmail.com... Sent (s748AgvS017005 Message accepted for delivery)
Closing connection to [127.0.0.1]
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Sendmail by default changes CNAME (DNS alias) to real name. You can turn it off in your sendmail but most sendmails beyond your control will do it anyway.

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