sMX Server has a Few Stuck Messages
This is a typical result of the sMX configuration. The sMX by default is a very passive mail configuration, it runs a basic Exim installation and does not run all the same filters that your cPanel machine does. This is in part to ensure the sMX does not block mail you otherwise would have wanted and had your regular mail server accepting, and also to enable it to act as a mail fail over for multiple cPanel machines, each of which may have different mail restrictions in place.
That said, the sMX will inherently accept certain messages that your cPanel server may have rejected. The filters, SpamAssassin policies, and BoxTrapper configurations are not in place on the sMX. As a result, if someone sends your server a message with the word “Viagra” in it, and it is offline, it will move to the next MX in your dns record which is the sMX sever. The sMX server says “thank you” and takes the message from that mail server, then queues it up and holds its for your server to get back online. When you server comes online it delivers it to you. However, you don't want it based on a regex filter match for the word “Viagra”, so your server tells the sMX server to go away by rejecting the mail.
The sMX doesn't have a way of delivering it back to the mail server it got it from, as it has already sent an accepted response to that server, so it sits on it. It will continue to try to deliver it to you again until the queue time out, at which point it will discard the message. It is also true that the sMX can pickup some mail even when you server is not online, it will do so if that record is tried as an alternative receipt to the mail when you main server rejects it. Generally this activity is minimal and consider negligible.
The benefits of keeping the sMX lite in regards to mail filtering has outweighed the adverse effect of delivering a few unwanted messages a few extra times. If in your implementation that is not the case, and something must be done about it you can add filters to Exim just as you would any other Exim installation. Your sMX is hosted on top our VPS platform and as such, you have root access to the machine and can make these changes. The sMX is not currently setup to import filter configurations from your cPanel machine, however this may be an option in future releases.