I need an external service to make sure my automatic notification emails are going out from my SMTP service

You are asking for a negative test. I.e. Notify if not received.
If you currently monitor the system, what are you checking.
You've not made clear what the issue is. Note that your situation might be that your mailings are seen as spam and your vps gets listed as a spam source, when that clears your messages flow.

I.e. All outgoing attempts have a 4xx error, defer message.

How quickly do you need to detect the issue? The question deals with how frequently do you need to receive an email from the server effectively saying, "hey, I am still here."
If you do not receive that email, within 5-10 minutes of the schedule internal. There is a likely issue on the vps' mail handling.

One option running a script is to generate what would be a text message to the phine via a direct SMTP session............

Yes, it's a negative test.  The problem is not emails not being received or accepted (spam).  The problem is sometimes the email stops being sent, for various reasons.  Monitoring my sending server is not enough.  I'd like to find a service that will expect to get emails regularly and if they don't show up within a defined period then it will notify me.

Do you currently have any monitoring setup?
Were you on the occasions that the issue existed, able to identify what the issue was I.e. SMTP server log shows outgoing connections denied, connections made, but the receiving server denied...

If the issue relates to being blacklisted, checking the lists mxtoolbox.com/blacklist
Will provide with a proactive test I.e. If listed, generate alert.
On the logs, an rbl based denial will reflect which rbl was consulted, such that your monitoring could do DNS query to validate whether your vps public ip/s are listed external to the vps generating alerts to you that would not succumb to vps blacklisting delaying the notification.

There are external monitoring tools/providers that could check different things including the negative events. Sone have agent based, meaning they connect to the agent, trigger an event I,e. Send email. If the email is received the test is successful, if the email is not received within a timeout period. The test fails and depending on your preference and their options, a second out of sequence attempt is made to confirm, if this test fails, a notification Is issued.

If you determine the cause or possible remedy, some could issue commands to correct.

One option on your vps, you could setup an external email account to which you will send a test from vps, and then check the external box to confirm an email was received.
In this case, the test is not conclusive if issue is with external resources.

my request is specific. i have gfi max focus and kaseya monitoring tools, there is no problem when the emails go out, I have dkim and spf setup on all sending domains, I simply want to know if there is a service that can alert me if it doesnt get a email for a period of time.

It is specific, but nothing in your question suggests that you have anything setup to monitor. Are you looking at an external monitor service? I provided you a way to test for email......
Are you able to identify in the logs the cause for the issue?

What resources do you have, nagios' zabbix, are some of available monitoring tools.
What is the running MTA on the VPS, sendmail, postfix, exim, qmail. Courier, etc?
Does your mail generating script logs events? This is yet another way to detect that the message is not being generated.
Might it need to restart periodically? How is the service running, is it triggered by cron, or is it an infinite loop service?
If it merely infinitely runs, but does not need, it merely needs to start at a prescheduled time/interval, running it as a cron might be better, but then you would need to monitor cron to make sure it is running............

the restart of the process will avoid the possible issue with a permanently running service that might have gotten stuck during a message transmission. Remote connection dropped, but it is stuck waiting for a response.

In what language is the service written? Does it catch if there is a failure in transmission, timeout in the connection, etc. that locks the service, versus abandoning the current and retrying.......to resend.
Having a local ?MTA versus trying to send through an external ............

Do you mean something like this service: http://www.resoftco.com/solutions-2/email-realtime-monitoring-usage-reporting-2/end-to-end-monitoring ?
I searched Google with these keywords: mail heartbeat service test.

A different angle: if you'd install a darn good mail server, would that solve your problem? A server that can be programmed easily to do the things you describe, with a proven track record, but not free?

Thank you both, there was good information from both of you.  The "Heartbeat" feature from resoftco was exactly what I was looking for.  But The nagios and Zabbix pointed me to some things that I will find very useful on other situations.