There are a couple of things that might be going on here.
- Your SmtpMail.SmtpServer has been black listed by recipients.
- Your server has been listed in an Open Relay Database, and you are being blacklisted.
- Your System.Web.Mail code is generating 8bit emails, and they are getting rejected/deleted at various gateways.
To check if you are generating 8 bit emails, view a generated emails' headers. If the Content-Transfer-Encoding header is set to 8 bit, this is probably the problem.
Check out How do I send non US-ASCII emails? for more information.