I don't think you need anything more than an email service if you mean sending newsletters by email.  You create web page like letters (html) and just mail them to people like any other kind of email.  Most email readers these days support full html.

I guess you WERE asking about mass email services.  

The only explicit newsletter I get is from Vicky Vette.  She runs a pretty big cam operation and sends out regular newsletters (well, ads.)  I suppose you could ask her what she uses.  The newsletters are full of porn clips, so there doesn't seem to be a content restriction of any sort.  She's pretty technically inclined due to her earlier career before launching into porn.
   
http://www.vickyathome.com/
[email protected]

Vallerie,

 
Take a look at Mailchimp. It free for basic service, easy use, allows users opt in and out, and integrate by link into website easy. I have a section in my website that allows visitors to subscribe to my updates in advance of posting on calendar. You can have them subscribe, you can manually add. You design your newsletter any way you want, then hit send. If they get tired getting newsletter, they can opt out easy.  

 
Best part, is that person that receive always only see his/her eMail. No accident that everyone see everyone and mess up anyone privacy.  

 
Christine

YMLP is the only one that I know of that won't shut you down for adult content.  Mailchimp has done so to many providers.

I can definitely attest to Mailchimp shutting you down for sending adult-related content through their mail service. I've been looking for another for so long, so thanks for the suggestion.