Brother, I haven't gone through each of the above mentioned sites, however, I have gone through most of them. So, I am sorry if some of the provided links are dead, but most of the provided links are working.

Bigmailbox.com, zzn.com, i-p.com provides you with a facility to give your users email facility. i-p.com is currently not accepting new sign-ups but bigmailbox.com and zzn.com are great. You should check them out yourself.