Recently I had various staff members reporting that emails they sent to others within the company (on the same domain connected via MS Exchange Server) weren't being delivered for hours or even days.

We managed to catch this happening a couple of times. The emails were sat in their 'sent items' folder but nothing had landed at the other end. Strangely, forwarding one of these emails to someone else freed them up and they finally landed. Also, leaving Outlook open for a few minutes also caused them to be released.

I didn't twig the solution to this until I realised all the users having problems sending were using Outlook 2003.

If you've used Outlook 2003 you'll notice there is an added 'Junkmail' folder. Previously, junkmail filtering between an Exchange Server and the end user hasn't been an option, because the end user isn't downloading the email. They just have a window into their email folder sat on the server. Outlook 2003 gets round this by adding a 'Cached Exchange' mode. When this is set as on (which it is by default) the user's email is downloaded to their machine. Outlook will connect to the server to do any send/receiving on a periodic basis. Another benefit of this is that if your Exchange server is offline your users can still read previously downloaded mail or cue up new mail to send.

The emails which weren't landing were being generated automatically by a CRM program. Outlook would open with the new email ready to send. The user then typically hit 'send' and immediately exited Outlook. So, the periodical connection to the server never had time to be established.

To fix this I have set Outlook to do an automatic send/receive upon exiting on each 2003 machine. At least I hope this is the fix - I'll let you know!