In very high volume email environments email servers may need to have more than 50,000 emails in the spool at any given time. However, SmarterMail's default setting is to load only 50,000 emails in the spool. If emails are throttled or delayed due to the remote servers not accepting emails into the spool, this can cause slow delivery for new message being created since the new emails cannot be loaded until existing emails are sent.
To solve this problem simply edit the mailconfig.xml file (default file path C:\Program Files (x86)\SmarterTools\SmarterMail\Service) and find "maxLoadedSpoolMessages". The default value is 50,000, but you can increase this number to allow more emails into the spool. NOTE: As you increase the max number of messages allowed into the spool your system will also increase the amount of system memory being used.