As an alternative to HTTP/HTTPS requests, you can "ping" checks by sending email messages to special email addresses.

Email address for pinging via email

By default, will consider any email received at the displayed address as a "success" signal. You can also configure to look for specific keywords in message's subject line to decide if the message is "success" or a "failure" signal. You can set up the keywords in the Filtering Rules dialog:

Setting filtering rules

Use Case: Newsletter Delivery Monitoring

Consider a cron job that runs weekly and sends weekly newsletters to a list of email addresses. You have already set up a check to get alerted when your cron job fails to run. But what you ultimately want to check is if your emails are getting sent and delivered.

The solution: set up another check, and add its email address to your list of recipient email addresses. Set its Period to 1 week. As long as your weekly email script runs correctly, and there are no email delivery issues, will regularly receive an email, and the check will stay up.