I've got this problem that only this one message gets stuck in the internet SMTP connector queue event sink errors preventing other messages from going out.
If I understand it correctly exchange creates instances of the message for every recipient and places that in the queue. So if there are 11 recipients in the To... box there should reflect 11 messages in the queue. But when I view the queue there are only 10 messages and retry deliveries are already on 12 retries. Freeze these 10 that retries with SMTP protocol event sink errors and my other mail goes through fine.
Runnig Exchange server 2003 Version 6.5 (Build 7638.2 : Service Pack 2) installed on a Windows Server Std 2003 SP2, with GFI MailEssentials 12.0 and GFI MailSecurity 10.0.