Improve email settings validation and UX (#3891)

BTCPayServer/Validation/MailboxAddressValidator.cs

@@ -0,0 +1,39 @@
+#nullable enable
+using System;
+using System.ComponentModel.DataAnnotations;
+using System.Diagnostics.CodeAnalysis;
+using System.Text.RegularExpressions;
+using MimeKit;
+
+namespace BTCPayServer
+{
+    /// <summary>
+    /// Validate address in the format "Firstname Lastname <blah@example.com>" See rfc822
+    /// </summary>
+    public class MailboxAddressValidator
+    {
+        static ParserOptions _options;
+        static MailboxAddressValidator()
+        {
+            _options = ParserOptions.Default.Clone();
+            _options.AllowAddressesWithoutDomain = false;
+        }
+        public static bool IsMailboxAddress(string? str)
+        {
+            return TryParse(str, out _);
+        }
+        public static MailboxAddress Parse(string? str)
+        {
+            if (!TryParse(str, out var mb))
+                throw new FormatException("Invalid mailbox address (rfc822)");
+            return mb;
+        }
+        public static bool TryParse(string? str, [MaybeNullWhen(false)] out MailboxAddress mailboxAddress)
+        {
+            mailboxAddress = null;
+            if (String.IsNullOrWhiteSpace(str))
+                return false;
+            return MailboxAddress.TryParse(_options, str, out mailboxAddress) && mailboxAddress is not null;
+        }
+    }
+}