Outgoing Email Issue

I have been using a motion detection application for the past five or six years that is setup to send an email with one or two .jpeg attachments when motion is detected to a cell phone (the cell phone receives these as SMS/MMS text messages) and to a gmail and yahoo account. As cox email security has changed thru the years this application was changed to reflect those changes with the latest being TLS and port 587. Every thing was working fine until about a week ago, the cell phone and the yahoo and gmail accounts started not getting all of the messages with the attachments. Got a message from cox to update the profile and change the password on the cox account that was used to send those messages with attachments, made those changes and I also changed the password for that cox account on the application. Since the changes were made to update that cox account, the sending of the emails with the attachments always indicated that the email was sent but the cell phone never receives those messages and the yahoo and gmail accounts receives them only once in awhile. I than conducted tests by using cox web mail to send emails with .jpeg attachments to the cell phone, to the yahoo and gmail accounts, the messages show as being sent and are also in the sent folder of the cox account used for this test. The cell phone never receives those test messages, the yahoo account did not receive the messages and the gmail account only received one out of two that were sent. I just did another test using a different cox account, same results.

Yes, I can send and receive emails from myemail.cox.net without many issues. Already did this previously. The cox email account was removed from the client device application, the client application was uninstalled, the system was powered off, three minutes later the system was powered up, the client application was reinstalled, the cox email credentials re-entered.
The client application sent a test email with .jpeg attachments to the cell phone, to the yahoo email user and to the gmail user. The client application shows that the email was sent successfully, but the cell did not receive it and both the yahoo and gmail users did not receive it. 
This client application has been sending emails with .jpeg attachments to a cell phone for more than five years with no issues until about one or two weeks ago.
Question - Is it possible the cox cyber security team is blocking my client application from sending emails to the cell phone and other users?

What IP address would that be? My cox IP address? What domain or host name are you referring to?

The client device is just a standard personal computer that is connected to the internet thru cox, the client application ( PYSoft Active Webcam) resides on it.

The client application has a standard web cam connected to it and can detect motion.

The client application just sits on the PC and does not connect to a specific domain.

This application is configured with cox smtp settings and using one of my cox email accounts to send out an email notification of when motion is detected with a .jpeg attachment, it does not send out a lot of email.

This PC has virus protection and anti-malware protection on it and scanned on almost a daily basis too ensure that it is clean, this PC is also not used for regular internet surfing.

This setup has worked for the past five or six years without any issues until about one or two weeks ago when I received a cox email to update the profile and change the password for the cox email account I was using for the application.

All indications from the application on this PC is that when motion is detected an email with a .jpeg attachment is sent but the recipients never receive it.

There are no error messages that indicate the email was not sent or that the smtp server was not found, everything indicates that smtp server is connected and that the email was sent successfully. 

So I ask again.

Did the cox cyber security team put the name of this application on some list to prevent it from sending out these email notifications?

If they did.

How can I get it removed from that list, since this application is not a spam generator?