This typically indicates that you should check your PHP error log for errors found there. Those errors can then provide more clues as to what the specific issue is. If you’re able to locate your PHP error log and share the errors from your log here. If unfamiliar with how to locate your PHP error log, please see this post here: How do I find my PHP error log?
Additionally, we provide some documentation on the configuration and troubleshooting of email, that might be a good first step:
https://docs.pkp.sfu.ca/admin-guide/en/deploy-email
-Roger
PKP Team