Well the DVR is acting like an email client (so Outlook, Thunderbird etc) you don't (usually) need to mess with router settings for outbound traffic. You wouldn't port forward or DMZ a PC to send mail for example.
Therefore I imagine your email provider has somehow changed how they accept your DVR's messages, this could be a list of things your password or server is incorrect or some new 2FA feature or a security settings or spam settings blocking a type of client.
You'll have to break down the problem to find out where in the chain sending the email fails.
Generally I wouldn't ever bother with emails from CCTV as it's unreliable and easy to defeat, more hassle than it's worth for the effort IMO.