jstedfast/MailKit · error · ArgumentException

The alias cannot be empty.

Error message

The alias cannot be empty.

What it means

SmtpClient.Expand/CreateExpandCommand throws ArgumentException when the alias is the empty string. An empty alias would produce the invalid command 'EXPN \r\n' and cannot be resolved by any SMTP server, so it is rejected up front.

Solutions

1. Check alias.Length > 0 (or !string.IsNullOrWhiteSpace) before calling Expand
2. Trim and validate user-supplied aliases; reject empty input with a user-facing error
3. Skip empty entries when iterating over a split alias list

Example fix

// before
foreach (var alias in input.Split(','))
    await smtpClient.ExpandAsync(alias);
// after
foreach (var alias in input.Split(',', StringSplitOptions.RemoveEmptyEntries | StringSplitOptions.TrimEntries))
    await smtpClient.ExpandAsync(alias);

if (string.IsNullOrWhiteSpace(alias))
    throw new ArgumentException("Alias must not be empty", nameof(alias));
await smtpClient.ExpandAsync(alias);

bool IsValidAlias(string a) => !string.IsNullOrWhiteSpace(a);

try {
    var list = await smtpClient.ExpandAsync(alias);
} catch (ArgumentException) {
    logger.LogWarning("EXPN called with empty alias");
}

When it happens

Trigger: Calling Expand/ExpandAsync with "" — e.g. a trimmed user input that was empty, or string.Split producing an empty element that is passed straight through.

Common situations: User submitted a blank mailing-list name; Split(',') on an input like "a,,b" yielding empty entries; config key present but set to empty string.

Understand the failure class

Background: "must not be empty", "cannot be empty" — required-field validation errors across open-source libraries — this error's family across 41 libraries.

Related errors

• The address cannot be empty.
• The address cannot contain newline characters.
• The alias cannot contain newline characters.
• No compatible authentication mechanisms found.
• No recipients have been specified.

AI-assisted analysis of jstedfast/MailKit@9d3859a785 (2026-09-15). Data as JSON: /api/errors/c4fc50a40f36ac29. Report an issue: GitHub.

Appendix: source

Thrown at MailKit/Net/Smtp/SmtpClient.cs:2681

		/// <exception cref="SmtpProtocolException">
		/// An SMTP protocol exception occurred.
		/// </exception>
		public override string Send (FormatOptions options, MimeMessage message, MailboxAddress sender, IEnumerable<MailboxAddress> recipients, CancellationToken cancellationToken = default, ITransferProgress? progress = null)
		{
			var rcpts = ValidateArguments (options, message, sender, recipients);

			return Send (options, message, sender, rcpts, cancellationToken, progress);
		}

		#endregion

		string CreateExpandCommand (string alias)
		{
			if (alias == null)
				throw new ArgumentNullException (nameof (alias));

			if (alias.Length == 0)
				throw new ArgumentException ("The alias cannot be empty.", nameof (alias));

			if (alias.IndexOfAny (NewLineCharacters) != -1)
				throw new ArgumentException ("The alias cannot contain newline characters.", nameof (alias));

			CheckDisposed ();

			if (!IsConnected)
				throw new ServiceNotConnectedException ("The SmtpClient is not connected.");

			return string.Format ("EXPN {0}\r\n", alias);
		}

		static InternetAddressList ParseExpandResponse (SmtpResponse response)
		{
			if (response.StatusCode != SmtpStatusCode.Ok)
				throw new SmtpCommandException (SmtpErrorCode.UnexpectedStatusCode, response.StatusCode, response.Response);

			var lines = response.Response.Split ('\n');

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