MM-60722 - don't allow multiple '@' in email (#28481)

* don't allow quoted strings in email

* don't allow multiple '@' in email

---------

Co-authored-by: Mattermost Build <build@mattermost.com>

server/public/model/utils.go

@@ -631,6 +631,12 @@ func IsValidEmail(email string) bool {
 		return false
 	}
 
+	// mail.ParseAddress accepts quoted strings for the address
+	// which can lead to sending to the wrong email address
+	// check for multiple '@' symbols and invalidate
+	if strings.Count(email, "@") > 1 {
+		return false
+	}
 	return true
 }