Code:
mwalling@youtoo:~$ host domain.com
domain.com has address 66.150.120.131
domain.com has address 216.34.94.184
domain.com mail is handled by 10 sentry.domainbank.com.
mwalling@youtoo:~$ telnet sentry.domainbank.com smtp
Trying 64.85.73.28...
Connected to sentry.domainbank.com.
Escape character is '^]'.
220 mailbucket.registerapi.com ESMTP
EHLO you.dontlike.us
250-mailbucket.registerapi.com
250-PIPELINING
250 8BITMIME
MAIL FROM: <postmaster@you.dontlike.us>
250 ok
RCPT TO: <postmaster@domain.com>
553 sorry, that domain isn't in my list of allowed rcpthosts (#5.7.1)
QUIT
221 mailbucket.registerapi.com
Connection closed by foreign host.
mwalling@youtoo:~$
I can connect to domain.com's MX just fine (ignoring the misconfiguration).
Without logs or configuration settings, it would be hard to see what is wrong.