There are new enhancements to SMTP support for .NET 4.0 - in the area of Unicode support and increased standards compliance, which is an important aspect in ensuring that legitimate emails do not get flagged as spam, as well as a few other useful features.
+ Decreased likelihood of being accidentally flagged as spam
+ Increased Unicode support
+ Clarification on setting header values
+ Multiple Reply-To addresses with the new property ReplyToList
+ Content Disposition time zones
+ Mail Address formats
One of the interesting thing that i noticed with Authentication is, “Some legacy clients do not advertise their AUTH mechanisms correctly. Normally, in the EHLO reply you would expect to see something like AUTH LOGIN NTLM advertising that this server supports LOGIN and NTLM authentication. Some legacy servers will instead reply with AUTH=LOGIN NTLM which is a violation of the RFC, however we have added support to SmtpClient for these legacy servers so authentication will work correctly should you encounter one.”
For more information, you can refer the following blogpost for detailed information on this.