You can link directly to an email message in IMP if you know the messages IMAP UID (available using http://php.net/imap-uid or by passing FT_UID to many functions if you are using PHP's imap extension) and the folder it is in. The url syntax is:
https://servername/horde/imp/message.php?mailbox=MAILBOX&index=UID
So to link to a message in a user's INBOX with UID 3 you would use:
https://servername/horde/imp/message.php?mailbox=INBOX&index=3