IIS/SMTP - emails are stuck in mailroot/Queue

I'm trying to send e-mails via SMTP within the IIS pickup directory. Unfortunately the e-mails are just going into the mailroot/queue folder and stay there. They never actually get sent.

Does anyone know why this would happen and a potential fix for the problem?

Had a similar problem with Files stuck in the queue. In IIS manager, SMTP Virtual Server > Properties > Delievery > Outbound Connections. The option for Limit number of connections to was checked and the value was 0. So it was configured to never make any outbound connections, causing the emails to never leave the server. I unchecked the option and restarted the SMTP server and all was well.

I had this problem today.

After restarting the 'Simple Mail Transfer Protocol (SMTP)' service, it started to work again.