Yep i’m in the same boat as well. I have to pay to use an smtp provider when I have my very own!!!
What the hell nextcloud. My Exchange 2019 enterprise server works perfectly for all of my send-mailmessage script, all my blat scripts, all these cheap POE CCTV cameras but NOOOOOOOOOOOOO internal nextcloud ohhh sorry A problem occurred while sending the email. Please revise your settings. (Error: Email could not be sent. Check your mail server log).