cannot send emails

[y-nk]

i'm trying to deploy the simplest image ever with:

FROM namshi/smtp
ENV MAILNAME some-domain-i-own.com
EXPOSE 25

which deploys correctly. i can ping the machine and telnet it. when using telnet:

HELO some-domain-i-own.com
250 some-domain-i-own.com Hello [171.xxx.yyy.zzz] [172.xxx.yyy.zzz]

and

EHLO some-domain-i-own.com
250 some-domain-i-own.com Hello [171.xxx.yyy.zzz] [172.xxx.yyy.zzz]
250-SIZE 52428800
250-8BITMIME
250-PRDR
250 HELP
MAIL FROM:<y_nk@some-domain-i-own.com>
250 OK
RCPT TO:<y_nk@gmail.com>
550 relay not permitted

Any idea what's wrong?