For a person whose DESK address is:
  JANE DOE /HP1234/XY

Ignore the subentity (XY) and use the form:
  jane_doe@hp1234.desk.hp.com

Send them a test message and tell them to make a note of the return
address, as forming internet addresses on DESK is a little more
complicated.  If there is an X.400 system between you and the DESK
person, what you get back may look like a very strange internet address,
but it generally works.

Notice the underscore between names.  Names can be first_last or
last_first, but first_last is easier to remember and get correct,
especially if they have initials in their name as in
fred_l_doe@hpatc1.desk.hp.com.  Be sure that the DESK address they give
you is exactly what is reported by DESK when they send a message to
themselves or look at the distribution list on a piece of mail the
recipient already has to verify the address.