I suggest we troubleshoot bottom, up. The error you are getting appears to indicate there are no recipients. Are you able to run a SQL query?
Something like: select * from eventummail_queue order by maq_iss_id desc limit 1; (Note my table prefix is eventum and you may need to replace or eliminate this prefix)
OR select * from eventummail_queue order by maq_iss_id = 2184; (The specific issue that is not emailing) If you want to obfuscate domain info, please just replace text (not delete) I am assuming no special characters/unicode in the email addresses?
Not sure why, the chat client ate the _ between eventum and mail_queue.