Hello Michael, I'm sorry to hear you're having spooling issues. Your best bet is to submit a ticket on this so we can review your logs in more detail I think. In the meantime, the majority of spool backups are typically caused by DNS lookup failures or temporary issues preventing SMTP from completing successfully. To this end I usually recommend doing two things to resolve these issues long term. First, head over to Settings>General and confirm you have at least one public DNS server (or DNS server outside of your network) set up here to ensure you have a fallback DNS server to use in the event your internal one goes down. I use these:

Next, delayed spam checks can cause these types of issues as well, and I recommend we look for any slow responding spam checks. Head over to Settings>Antispam and look for any entries under RBLs/URIBLs showing high response times and disable them to see if this resolves your spooling issues. Let me know how it goes as you find out more on this. Have a good one!

Try to disable TLS on SMTP Out:

Note that using public dns servers will most likely break RBL lookups. So be careful with that.