This is different to IMS.
SMTP of MAPI voicemail email sending (the request you are receiving) is used in conjunction with the Manager settings for copy, forward and alert for voicemail to emails. This is used for one way only, sending.
IMS allows syncronization of voicemails and voicemail emails with the use of your exchange server. For example:
A voicemail is left, which gets put into your exchange client email box, a custom form is used with outlook which allows you to play the message through your phone or on your computers speakers.
If you delete the voicemail from your email, it deletes it from your voicemail box aswell and vise versa if you delete it from your voicemail box, it deletes it from your email.
IMS can attach the wav file to the email or leave it on the voicemail server. The SMTP and MAPI can only attach it the wav file which will put more load on email servers.
You do not have to use it, its quite a good feature though.