I've found that the best way to administer your email accounts is via the SmarterMail web interface, which should be accessible at mail.yourdomain.com.
There were some email features (mailing list, etc) that just didn't work correctly when administered via DNP....this may be one of them.
To answer two of your other questions:
1. The Reply email field is only necessary if you want replies to emails from that email account sent to a different email account. For example, you may use mark@mawhitfield.net to send your emails, but want all replies to those emails to go to sales@mawhitfield.net. Leaving the Reply field empty is the standard practice.
2. Regarding SMTP: https://www.ultimasupport.net/Customer/KBArticle.aspx?articleid=1
Ideally you should use your ISP's SMTP address for sending email from your email client. However, if you need to send emails from your .Net application: https://www.ultimasupport.net/Customer/KBArticle.aspx?articleid=11
William Eaton, MCSE
iTech Computer Solutions, LLC
www.itechcs.com