Recently gmail have had issues with their SMTP server and this has caused a lot of problems with e-mails being sent directly from your Hikvision or Alliance DVR.
Essentially there is a problem with smtp.gmail.com as the address is currently not resolving properly.
To resolve this issue you can update you SMTP server settings in your DVR to be the IP address of Googles SMTP server which is 18.104.22.168.
This should resolve any issues you are having when sending emails from your Hikvision or Alliance DVR
If you need any further information on setting up your DVR to send e-mails please watch the how to video below.