I don't faced this kind of error before

i try to send an email from my local application with asp.net using the credentials of the server
host port email password
before was working but now not

this is what i see when i try to send an email ==> The remote certificate is not valid according to the validation procedure.
what should i do ???

Add the following to your code:

ServicePointManager.ServerCertificateValidationCallback += (sender, certificate, chain, sslPolicyErrors) => true;

This disables certificate verification. BUT BE WARNED RIGHT NOW: Doing this is a HUGE security risk. I recommend getting a valid SSL cert. (Let’s Encrypt, GoDaddy, etc).