IMAP is perfectly capable of doing this task. Exchange may not be (I am not familiar enough with its implementation of the protocol to say). It is absolutely possible to grant a user read-only access to other people's mailboxes (see, for example, IMAP::Admin, Mail::IMAPTalk). The Mail::IMAPTalk module will allow downloading attachments, providing unique ids and changing folders (there's an example in the POD of looking at another user's mailbox):

  $imap = Mail::IMAPTalk->new(
            Server => 'foo.com',
            Port => 143,
            Username => 'joebloggs',
            Password => 'mypassword',
            Separator => '.',
            RootFolder => 'inbox',
            CaseInsensitive => 1)
          || die "Connection to foo.com failed. Reason: $@";

  $imap->select('user.john'); # Selects 'user.john'

  my $Res = $imap->fetch('1:*', 'all');
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Updated: Here's the example for changing folder access:

# Get full access for user 'joe' on his own folder 
$imap->setacl('user.joe','joe','lrswipcda') || die "IMAP error: $@"; 
# Remove write, insert, post, create, delete access for user 'andrew'
$imap->setacl('user.joe','andrew','-wipcd') || die "IMAP error: $@";
# Add lookup, read, keep unseen information for user 'paul' 
$imap->setacl('user.joe','paul','+lrs') || die "IMAP error: $@";
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