Full Slate follows best practices to increase inbox delivery, and we monitor our sender reputation to ensure our servers aren't blacklisted. That said, spam filters are tricky, and sometimes legitimate emails end up in the junk folder. The algorithms are constantly changing, but this article has a good overview of "common mistakes" that can cause messages to be mistakenly flagged as spam:
There are more helpful tips on how to avoid spam filters here:
You might try editing the subject or body of your emails under Messages > Templates to follow the rules laid out in these articles.