Bind 9.10 - add CNAME record value change

[nw2619]

Bind 9.10 - add CNAME record value changed automatically

I need to add a CNAME "email.mail.acueductochiguala.com", with the value "mailgun.org". The problem is that when I add it, the value automatically adds the domain, example:
mailgun.org.acueductochiguala.com
and I just need the "value" equals to "mailgun.org", why does this happen ?, what can I do to solve it?

Note: Add the register file

$ttl 38400
acueductochiguala.com.	IN	SOA	acueductochiguala.com. nw2619-web.yahoo.es. (
			1524625512
			10800
			3600
			604800
			38400 )
acueductochiguala.com.	IN	NS	acueductochiguala.com.
acueductochiguala.com.	IN	A	35.185.119.47
[URL unfurl="true"]www.acueductochiguala.com.[/URL]	IN	A	35.185.119.47
email.mail.acueductochiguala.com.	IN	CNAME	mailgun.org
mail.acueductochiguala.com.	IN	TXT	"v=spf1 include:mailgun.org ~all"
smtp._domainkey.mail.acueductochiguala.com.	IN	TXT	"k=rsa; p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQC2CwJ9ktcURiZ2aklxiXmaJc7DsnCvnNE6tYS4k7vlfdx2RhZxeR1A6GYhmMiSfL9axksicwv6kHUb4I01waZn/aoq+L3WCQw973bfzDNnHJybQZkPa46vtSjLVFhgIRolIAKb5/Wv963RVSVNz47aKw7NGvu4jmV7Ho3vR4lLHwIDAQAB"
mail.acueductochiguala.com.	IN	MX	10 mxa.mailgun.org
mail.acueductochiguala.com.	IN	MX	10 mxb.mailgun.org

Thanks!.