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I want to send mail to the client in nopcommerce 3.70 but it is getting the error while sending the mail. It shows "The SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.5.1 Authentication Required." when send email to the client emailid.

Here is my configuration for the emailID in admin panel of nopcommerce

Email address :
Email display name : extra mail
Host :
Port : 587
User : extra mail
Password : xxxxxxxxxx (10 digit of password containing numeric as well as alphabet)
SSL : Enable

I saw this related to this question in this forum and also I tried but showing the same issue.

Thank You.

What I have tried:

I tried many of things like turn on the
"Allow less secure apps" 
option in gmail account etc by refering related to question website but still shows the same issue in nopcommerce.
Updated 7-Nov-17 22:48pm

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,
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 - nopCommerce[^]
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Finally got an answer and i.e. want to take the permission from gmail account that it is our mail only[as per the security propose]. This permission link "Someone just used your password to try to sign in to your account. Google blocked them, but you should check what happened." has send by gmail in our access gmail email account and it is happening when we try to send mail by any application and hence from that by taken permission from gmail account we can access to send mail from our gmail account to other email account. 
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