Question : dsn=5.0.0, stat=Service unavailable

I have a mailing list at lists.mydomain.com and wanted to subscribe to it with an
email of [email protected] but the confirmation message didn't reach me. I
checked the log files of lists.mydomain.com server and can see

Jan  1 02:10:20 server sendmail[4248]: k0s41sbbp04546: to=,
delay=00:00:00, xdelay=00:00:00,mailer=esmtp, pri=229720,
relay=control.mynet.net. [63.175.195.228], dsn=5.0.0, stat=Service
unavailable

where
;; ANSWER SECTION:
mydomain.com.               1165    IN      MX      10 control.mynet.net.


I can subscribe using other emails so I guessed the problem is only with emails @mydomain.com and I checked and found that those emails @mydomain.com works. I was confused where the problem lies!

Answer : dsn=5.0.0, stat=Service unavailable

1) Could you publish email "bounce" message you have received?
[ It usually contains more hints ]

2) You can send a test message to [email protected] with SMTP session tracking

As root (sendmail-8.12+) execute:
( echo subject: test; echo ) | sendmail -v -oi [email protected]