I have a client that is trying to connect their payroll system to my email server. Their payroll company uses EmailArchitect library to send direct deposit advices as email attachments. It uses the same routine as in the example in  https://www.emailarchitect.net/easendmail/ex/b/13.aspx#vb6-send-html-email-example

They get an authentication error. 

I have confirmed the server name, and username and password, and tried both ports 587 and 645