This is one of the most basic problems with SMTP configurations. Primarily, your solution should've worked, but it didn't. Because the error says,
Quote:
5.5.1 Authentication Required.
This requires authentication, not a secure channel. In this case, I would recommend checking the username/password combination first. If they are correct, only then "Less secure apps" option can be set.
send request
|- check require authentication
|- read credentials
|- verify credentials
|- check app condition
|- send OK
|- fail
|- fail
|- fail
Of course this is mere exaggeration, but this is how things work so if you cannot fix the previous step your app will not be able to utilize the services. Also, on the Internet you will find other solutions to this problem, such as,
* Weak password
* Insecure channel (TLS?)
* Less trusted app
* Google preventing sign-in (login to your account and see the activity)
* Much more.
Thus, if you really want to work out with this, or are using mass emails, why don't you use an email service provider? There are a lot of service providers out there, and they do not require such a complex setup before sending emails — just a token.
Read this article where I discuss (almost) everything you need to know while programming SMTP apps;
Universal SMTP code to send emails in .NET apps[
^]. However, written in C#, it can still give you an idea of what an SMTP is expecting from you and you can do the same in other frameworks.
Otherwise, it might be just with Nopcommerce, read this,
[SOLVED] 5.5.1 Authentication Required with smtp.gmail.com - nopCommerce[
^]