Pro: alerts by email and webhook

For free, a failed email shows a red notice in the dashboard until you dismiss it. With Pro, Mail > Alerts also sends the alert outside the dashboard:

• Alert email: it does not go through your SMTP server, since that may be what is broken. It leaves with the PHP mail of the hosting and may land in spam: add the address to your contacts.
• Webhook: paste an incoming webhook of Slack, Discord, Google Chat or Microsoft Teams. Mail posts JSON with the message in text (and content for Discord), plus error, to, subject and site.
• At most one alert every: if many emails fail in a row you get one alert, not one per email.

Click Send a test alert to check both.