IIS/SMTP on Windows Server 2012 – Emails stuck in Queue folder

I have installed IIS 6 on a Windows Server 2012.

I've configured the Virtual SMTP using my Gmail account. It is a quite classic scenario well described also on Technet docs, there are plenty of discussion also here on ServerFault.

I have also logged in to my Gmail account from this server to allow unsafe application. Another important thing is that I don't have two factor authentication enabled on Gmail.

SMTP configuration is:

Outbound Security

Basic Authentication with my Gmail account

TLS checked

Outbound Connection

TCP Port: 587

Advanced

FQDN: servername.local

Smart host: smtp.gmail.com

I've "Limit number of connection" property not checked under "General" tab.

Then I have created a small .txt file with a simple email, copied it to Pickup folder. The SMTP service works fine, but the email remains stuck in Queue folder.

So I've checked the Windows Event Viewer (Application and System logs), but I did not find useful information.

Also enabled the SMTP log, but no problems are reported.

Any suggestion?