Windows 2008R2, latest updates with SmarterMail Version - 14.3.5752
I just started to experience this issue the last several days with no apparent configuration changes. Emails will be delivered and they show as delivered in the Spool but they are not removed form the Spool. This then "clogs" up other deliveries and causes extended delivery delays for mail not delivered. If I delete the delivered mail everything starts moving along as normal again. I am not 100% sure but one of the triggers for this may be users emailing more than one recipient in an email.