[Serusers] SIP MESSAGE method

At some point in the not too distant past, I used to bounce SIP MESSAGEs back and forth from UA to UA on our server. Now, it no longer seems to work (get an error 513 -- message too big error... which is kind of a default for all unhandled SIP methods), and I'm not entirely sure how to go about fixing it. Can someone explain how a MESSAGE should be handled and maybe give me a hint as to why it used to work but does no longer (I know that's pretty vague, but just some ideas of what I might look at to point me in the right direction would be good)? Meanwhile I'll fiddle, but I don't seem to be getting anywhere. N.