When mails are stuck in Exchange Queue, you should check it’s not causing by your Anti-virus tool. The latest generation Anti-Virus tools hook themselves at SMTP service, so they can scan every mails passing through at SMTP level at Exchange.
When mails are processed by the Exchange SMTP service, it places in SMTP Table first for processing. SMTP Table is nothing but Mail Folder in a SMTP Mailbox, usually named SMTP (ServerName-UniqueID). Your Anti-virus software scans the mails in SMTP Temp Table aka mail folder, then it gets released.
To turn off the Anti-virus software on your Exchange server to troubleshoot the mail flow issues,
1. Stop the Anti virus software services
Open the Services console (start –> Run, type "services.msc"). Turn of all the Exchange Mail related Anti-virus services.
2. Disable the SMTP Event registration of your Anti-Virus
You need CDO Event Registration Tool 2.01 to unhook the Anti-virus. (Thanks TurboGeeks for the free utility). Find your Antivirus hook and you can select and stop it.
Don’t forgot to start the Anti-virus services and hooks after you resolve your mail-flow issues.