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Sucessfull STMP from personal gmail, issues sending via Business gSuite gmail account: .NET app 1 Recommended Answer 4 Replies 29 Upvotes
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Sucessfull STMP send from personal gmail, issues sending via Business gSuite gmail account: .NET app

We have successfully sent emails (for the past 6 months) from a personal email account via a .NET application. This is for internal testing only and is not spamming or email marketing. We typically send about 5-10 test emails to ourselves per week.  For this testing email address we set up two factor authentication and an application password as recommended per the google documentation. This has been very simple and is well documented. 

We have now migrated to a business gSuite gmail account.  We are looking to keep our application the same, but migrate to sending mail from our business gSuite gmail account. Per documentation recommendations we are looking to use the gmail smtp relay service if required.  I have setup the two factor authentication for the email address we are trying to use (from our gSuite business account, of which I am an admin), and also created an application password. In addition to this we have white listed our test location ipaddress for smtp and set smtp to available to all users on our domain emails. We are using the new application password in our application. 

It appears that using the same code that has been working for the last half a year with the new gSuite email address does not work. It's throwing an authentication error.  We have been successfully using Host: smtp.gmail.com, Port: 587, and Enable SSL: true for the past 6 months.  When switching the new gSuite email address we get the following error:  

"The SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.7.0 Authentication Required."  

I understand that this means our connection was not authenticated.  We believe we are following all recommendations per documentation.  Is there additional documentation or support resources that can help us with this?  Is there an additional authentication step that is not documented? It seems we are not the only one with this issue. A user had a similar issue last year and was refereed to the gSuite community: https://support.google.com/accounts/thread/10963564?hl=en&authuser=7 

I was not able to find any additional support there. Any additional support would be much appreciated.  

Cheers!
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For this new gSuite email account we have been using the fully qualified domain name of SMTP service:  
smtp-relay.gmail.com

Are there advanced security features that we have to enable through the google admin interface? We can not find any additional documentation outside of: https://support.google.com/a/answer/176600?hl=en&authuser=7

We are trying to authenticating with the application password specified in the user account.
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Rob,
Thank you for the reply. 

We have tested with the smtp relay server and had already setup smtp relay service from our google admin account.  We still get the same error. 

We had A support tech say that we may need to have less secure apps turned on for this email address.  Is that correct?  We want to follow your best practices and less secure apps are not recommended.  We would like to not allow less secure apps, but rather to use an application password (this is how we are currently sending email from our personal gmail account).  Is that possible with gSuite? Or do we need to have less secure apps turned on for the gSuite address that is submitting email from our application? We were told OAuth2.0 is an option, but we do not yet have that implemented.
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SMTP relay service does not require auth if setup correctly. You can just send with a valid email address and permitted IP and it will accept and send the email.

There is no need to less secure passwords.
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