Currently the mail: tag allows attributes such as to, from and cc to be set 
using either attributes or subtags (addrecipient etc). But it does not allow 
Server to be set - it looks mostly like an oversight, not a deliberate omission.

Subtags have proven to be extremely useful due to the nature of runtime 
evaluated custom tag attributes. Such attributes may contain only java and not 
further custom tags. For example, the following is not possible:

<mt:mail from="<mytags:something/>".....etc

However this may be achieved using subtags:

<mt:mail>
  <mt:from><mytags:something/></mt:from>
...

So to summarize: Please make a tag which allows the Server to be specified. It 
should be fairly trivial.