Hi,
I join your conversation, because I had the same problem and I was able to fix it.
First:
You need the new Firmware 5.4.0.
You can get it here:
http://www.hikvisioneurope.com/portal/index.php?dir=Product Firmware/Front-ends/01--IPC/R0 platform(2xx2)/
After the installation you have to enter the following in the email-Tab:
SMTP Server : smtp.gmail.com
SMTP Port : 587
Email Encryption : TLS
STARTTLS: enabled
Authentication: enabled
Username : (gmail address)
Password : (gmail app password)
Important: You have to create a specific gmail-app-password in the security-settings of your gmail-Account.
And tadaa, it works again.