Do you still experience this error? Manager is now including the latest version of .NET Core (5.0) so chances are if this was an issue caused by early version of .NET Core, it could be resolved by now.
Sadly no. I made some changes to the settings as well and it appears to send but never arrives at the recipient. So I don’t even have an error message to go off.
My email provider has washed their hands of me too.
If you get no error from Manager, this means Manager has received confirmation the email has been successfully accepted by your SMTP server.
I don’t know who is your email provider but it’s best to use company that is specializing in this (Mailgun, Fastmail, Outlook or even Gmail) rather than someone who doesn’t consider email service as their core competency (e.g. ISPs)