I have a suggestion for a new extension. Mail Uptime Monitoring. This would be very different from SMTP uptime monitoring.
You would need to provide a custom email on your Dashboard site’s server and give MainWP that account to monitor. This email account would be shared to the child sites who would send a regular email once (twice, thrice, etc.) daily (weekly, monthly). They would include some markers in the email/subject to show what child site/date/etc. The Dashboard site would track the incoming emails and validate them against what is expected and in this method it would ensure that each child site’s outgoing email is still working. It would delete each email once validated.
This would work for servers with their own email or those using WP SMTP to send outgoing emails via other servers.
If some email fails to validate, then trigger appropriate errors in the dashboard or email the admin.
I think the use cases are fairly obvious!