Improve email settings validation and UX (#3891)

BTCPayServer/Validation/MailboxAddressAttribute.cs

@@ -0,0 +1,25 @@
+using System.ComponentModel.DataAnnotations;
+
+namespace BTCPayServer.Validation
+{
+    /// <summary>
+    /// Validate address in the format "Firstname Lastname <blah@example.com>" See rfc822
+    /// </summary>
+    public class MailboxAddressAttribute : ValidationAttribute
+    {
+        public MailboxAddressAttribute()
+        {
+            ErrorMessage = ErrorMessageConst;
+        }
+        public const string ErrorMessageConst = "Invalid mailbox address. Some valid examples are: 'test@example.com' or 'Firstname Lastname <test@example.com>'";
+        protected override ValidationResult IsValid(object value, ValidationContext validationContext)
+        {
+            if (value is null)
+                return ValidationResult.Success;
+            var str = value as string;
+            if (MailboxAddressValidator.IsMailboxAddress(str))
+                return ValidationResult.Success;
+            return new ValidationResult(ErrorMessage);
+        }
+    }
+}