Email can’t send to my mail server

My company have two email system, one is using Lotus Domino, another is Sandmail. Lotus is under the main domain(mydomain.com.sg) & the sandmail is under the sub doamin(sub.mydomain.com.sg). DNS is host by ourself, with two MX record, one pointing to the Lotus, & the other pointing to the Sandmail.
The problem now is some of the mail server(mostly in the US) can’t send mail to the Sandmail server. They will get the error “doamin unreachable”.