What are the benefits of using email marketing services instead of using email server for delivering newsletter?
If you mean using email marketing services like 'Get Response' or 'AWebber', I'll say the following features are the benefits:
1. You have access to templates to build opt in forms and squeeze pages. That is if you do not have a plugin to do so, because squeeze pages plugins give you more options and flexibility than what marketing services generally offer.
2. You can also get newsletter and email templates that gives a more professional look.
3. You have a choice of sending emails and newsletters in HTML format - again having a more professional look.
4. Recently 'Get Response' is offering a free list building training program presented by Pam Neely, which I think is a huge benefit.