.. hmm, it would seem that MYOB are just as inept in this area as in most others (in my experience); its yet another triumph of marketing over content ... try turning off the SMTP proxy in the server manager and using your ISP's SMTP server as the outgoing server in MYOB (tho the ISP may be just as intolerant).
Better would be to use your service contract to contact MYOB and tell them about the problem; the level of abuse you level at them for being so silly is up to you.
SME adheres rather strictly to most, if not all, the mail RFC's and as such can be intolerant of programs that are poorly written.
HTH