telnet internalmailserver.mydomain.com 25
220 Ironportblabla.mydomain.com ESMTP
HELO
250 Ironportblabla.mydomain.com
MAIL FROM:test@gmail.com
250 sender <test@gmail.com> ok
RCPT TO:myemailaddress@mydomain.com
250 recipient <myemailaddress@mydomain.com> ok
DATA
354 go ahead
TEST
.
250 ok: Message 123456 accepted