Hello,
I have Two separate user Id's e-mail and I have maintained same e-mail address for both and also successfully Subscribed to tasks.
I suggest please check Fine Tuning >"E-Mail and Fax Settings" > "Allowed Sender E-Mail Domains" and make sure the Domain of your e-mail is maintained in the system. (only business approved domain are allowed to send mails out that is why there is this setting).
Best regards
Nick