IIS/SMTP - emails are stuck in mailroot/Queue

Recently I was working on sending mail using smtp server where I stuck on below error:

IIS/SMTP - emails are stuck in mailroot/Queue

Here are few solution that were tried and might help you:

• Check password and username in SMTP server

• Check Pop3 mail setting (require enable) in receiver/ sender mail settings. 

• SMTP Virtual Server > Properties > Delievery > Outbound Connections. The option for Limit number of connections to was checked and the value was 20. 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.

• 'Simple Mail Transfer Protocol (SMTP)' service might be stuck. Try restarting service.

• If you are using SharePoint, check running jobs (Microsoft incoming mail). If its is stuck, run it manually.

• Open IIS 6.0. Right click on SMTP Server Properties. Go to delivery tab on Outbound Connections, fill in TCP Port to 25( if its 587).