I've successfully

I've successfully sent 36 email newsletters using this plugin from this site over the past 4 years.
It's a bit seasonal, so I haven't sent one since September.
I've just tried and all 700 bounced, and a test message gives this error message: "Failed to send test email! Please check your outgoing settings and server config to see if selected ports are open. Error details: SMTP Error. "
Since September all I can think that has changed is a new theme (Divi) and an upgrade to PHP7