Same problem here. Haven't had an email report since the 3rd March and only sporadic alerts. Radarbox initially notifies email success, then several moments later "Message sending failed. Please check background software (antivirus/firewall) and try again later"
Nothing has changed in my settings but I rechecked and reentered mail details etc. No Joy.
I am pleased, if that's the right way to look at it, that others are having this problem too. I was at a loss.