Sounds like maybe your CA (Certificate Authority) is missing or incorrect? I would double check that your email service credentials and certificates are all up to date in either the project settings locally, or through the sprintr environment if you are running in accp/prod. If it had been working before and you changed nothing, then there is probably a change on the service side.
I would also try a different browser when testing the app just in case.
As Luke has suggested, it sounds like the root certificate your certificate relies on may have expired and you would need to update this.
Do you know who issued the certificate?