I wouldn't respond to unknown sender for several reason, especially security. Often those who sent e mail, text, etc would spoofing the E mail or text number and assume it is not exist by no response. They would look for those who response and unfortunately sometimes they can trace back to your machine and try to crack it open. They need response because often it comes with IP address where it came from then track back. Other reason, so with response, they can sell my information to spammer or marketers. Returning e mail is close to give them permission to do such. I don't want that.
This applies to videophone, where we see "TEST", or something like that. I am not sure why this happens however I can guess that they are looking for a valid IP address to use as proxy server for any criminal activity or try to crack victim's computer system. I would never try to call back to the unknowns.
Just think this way, a computer hackers or virus writers would send trillions of E mail, and got just a million of responses, would they even bother the rest of UN-responders?, you could guess it easily.