SendGrid offers a free plan which provides 40,000 emails a month, which should be more than enough for most MainWP dashboard sites. Using a transactional email provider like SendGrid, SparkPost or SendinBlue.
After you have signed up for a free plan you will be in the main app.
On your MainWP dashboard site look for SendGrid and install it as a new plugin on the site.
Go back to the SendGrid app and then click on Settings > API Key then click on the Create API Key button.
Create the API, then on the next screen the API key will be shown, this is what you will need to copy into the settings of the SendGrid plugin.
Copy the API key and also enter the name, sending address, reply address and set the content/type, the default is text/plain, change this to text/html, click on the update settings button.
When you go back to the main dashboard in wp-admin, you will now notice a new admin dashboard widget called SendGrid WordPress Statistics, which will show you the following.