Email sent using .NET rejected from different domains and mailservers

I am sending automated emails from several of my .NET sites - such as invitations for events, etc.. to a list of registered users, and I am hosting at DiscountASP.Net. I am using Advanced Intellect ASPNetEmail, but I am probably going to switch to ASP.NET 2.0 built in class, System.Net.Mail, just to standardize things for my sites.

I am having issues (besides the known AOL.com automatic email rejection problems) when sending out automated emails, where some domains are rejecting the emails or targetting them as spam. As well, I am having much more success with plain text email than HTML, although there's no reason I can figure out for this difference (my HTML emails are fairly simple).

Is there any way for me to:

1. Improve the construction of my email, in order to  ensure more emails get through, and

2. Improve the construction of my email, in order to allow more people to receive HTML or embedded emails ?

Thanks, Peter