When pressing an email link in the email column the email client is opened with a pulse ID placed in the Cc box.
After cutting and pasting into the Bcc box (already another feature request to avoid this needless step) you send the email. If the email is not a user on your account then the pulse update does not occur and you receive a bounce back email saying it is an unrecognised email.
I see the purpose of this is to avoid spam updated from unwanted emails. Someone can easily get your pulse update email from the Cc box if it is left there and send you unwanted updates. I am not suggesting getting rid of this limitation.
My suggestion and feature request would be to have the option of adding a list of recognised emails to your account.
A current work around for this is to use a dummy account with the email address you want to send emails from. This will be limited for some users depending on how many spare spots you have on your subscription.