Evolution notoriously change the menu contents
(compare the list of commands present in 'Tools'
menu while one previews sumamry and while he or
she views mail). Such a design causes user
confusion and 
is considered to be violation of usability rules
(see for instance 'GUI Bloopers' by Jeff Johnson).
According to the same rules, it is OK to
add/remove main menus and to disable/enable menu
items.

Recommendation: reconsider the evolution main menu
structure and make it more constant:

1) Consider moving the items valid only in one
mode into the separate menu, which can be
displayed or not depending on mode (for instance,
move all the message management functions from
'Edit' and 'View' to new menu 'Message', instead
of changing the contents of 'Tools' menu,
create new menu named 'Mail Folders' and move
there all the mailfolder actions which pollute
Tools currently).

2) Where it is impossible, leave the options
not valid in some mode in the menu - and disable them.

3) Consider supporting some options in all modes
instead of only one mode (for intance, 'Mail
settings...' could be active always)

In fact, I feel that the current menu structure
has been created without deeper design, just using
'we have some function, so let's add it somewhere'
idiom. There are also problems with unclear or not
suitable names (please find any tool in Tools
menu...) and unclear criteria of distributing
actions between menus (mail processing functions
randomly distributed between Edit, View and
Actions). Evolution would really benefit from the
systematical menu structure re-design.

I do not consider myself to be usability expert,
but in case you express intent to implement such a
changes, I could probably prepare more detailed
set of suggestions. Nevertheless, if it would be
possible to get some experienced designer for this
task, I would recommend it.