- Check the rules for the mailbox to ensure that the message is not being sent to a folder based on rules.
- Verify that the mailbox is not full and has enough room to receive emails.
- Check the settings of the folder, both at the destination and at the source to verify that the message is being posted correctly.
- Verify that the message is not being blocked – either by server settings, by user settings, or by email security.
- Run the email queue viewer to check the email delivery process parts of the process.
- Check for any errors in the application / system logs associated with the email delivery process.
- Check any virus scanners that may be intercepting the email message.