I just tried to get it working and failed. After looking at the code (lib/python/Products/MailHost/*.py), it seems that the smtphost is never evaluated as an expression, preventing its use in the way you are attempting. Bummer.
A mailhost and acquisition would (I think) do what you require. However, the title of this thread leads me to believe this won't work with your particular setup (Exchange). Boo.
Well, the way I got around it was to define a method called mail_sender; <dtml-sendmail smtphost="my.smtp.host"> To: <dtml-var mail_to> From: <dtml-var mail_from> Subject: <dtml-var mail_subject> <dtml-var mail_text> </dtml-sendmail> ...and then call it as follows; <dtml-with "_.namespace(mail_to='david@connmed.co.uk', mail_from='Zaphod X. Beeblebrox <zaphod@hhgttg.com>', mail_subject='Another Test', mail_text='you\'re a hoopy frood who really knows where his towel is.')"> <dtml-var mail_sender> </dtml-with> ...which seems to work OK. Bit clunky though. Thanks for taking the time to look into this for me -- much appreciated. Cheers David