If you are using a Gmail email account that is only used for sending SMTP email alerts from your CCTV system but not used for sending real email this could be the cause of the issue you are experiencing.
Gmail require you log in to the email service and send an email every 3 days to keep the account active. SMTP emails do not class as logging in so the Gmail account suspends itself due to inactivity.
To resolve this either ensure you keep your Gmail account from being suspended by logging in to the account ever 2 days and sending an email to a live valid email address.
Alternatively use an SMTP server from a paid for email service E.G. Private domain name hosting company, free ISP provided email account etc.