The tag <mt:header .../> performs a setHeader what resets the headers previously defined with the same name. It should perform a addHeader to allow defining more than one value for the same header name (Optionnaly a "reset" attribute could be added, but it doesn't make sense to me: why set a value first to overwrite it just after?)
Created attachment 10693 [details] Patch against SendTag class rev 1.18
Fixed in the 20040308 nightly build.