Sigh. Ok, I can understand frustration and quick answers out of frustration, I'm married, but when something important happens in life, one realizes how trivial aggravation on a forum really is.
(On the other hand, the mail command is a pain, I recommend mutt for that.)
If you don't want to read them....
Well, check if sendmail or postfix is runnning.
If you get an answer, that means yes, it is running. If you just go back to a command prompt it means it isn't, and my answer is equally irrelevant.
Just to be sure, if it goes back to a command prompt, try
If it still goes back to a command prompt, well, you can stop reading. Otherwise, if one or the other of those gives you a line with a number, or perhaps two lines. Most likely is sendmail, so then you can do
systemctl disable sendmail.service
as root or with sudo.
If you got nothing with sendmail but got something with pgrep master, then use postfix.service instead of sendmail.service
(Not sure about that--postfix might just show up as sendmail, but regardless, in that case, you'll get an answer with the first pgrep)
Next time you reboot, it will stop sendmail, and then you shouldn't get those messages, I think, but I'm not guaranteeing..
As for this answer, if you don't like it, as I said, I'm married, so you can be as abusive as you want--that stuff rolls right off, but as I tell my wife when she's throwing things at me, it's not the best way to get help.