There are many reasons that can cause this problem.
- The email address you are sending to is invalid or out of service.
- The email content contains one or more Bare LF (Line Feed) character(s).
According to RFP 822bis, every line in your mail message is required to end with CR LF (Carriage Return / Line Feed). IIS 6.0 SMTP server is much stricter than its predecessor in restricting Bare LF, all email containing Bare LF will be held in queue.
To resolve this problem, check your application for Bare LF.
In VB (both ASP & ASP.net), make sure to use vbCRLF rather than vbLF
In C#, make sure you use \n\r rather than just \r or \n alone.
