Allow me to jump into this conversation and provide a few details about the authentication feature when setting up email in QuickBooks Desktop.
Did you enter the Simple Mail Transfer Protocol (SMTP) port number in QuickBooks? You can check the Intuit's supported list of Internet Service Provider (ISP) in the article above provided by my peer, AdrianRose_A.
If your email provider isn't supported, I suggest reaching out to them. They will be able to help configure the SMTP's server settings to allow QuickBooks to send emails.
This should point you in the right direction. Please let me know if you have any other concerns. I'm always around to help.