jstedfast/MailKit · error · InvalidOperationException

No recipients have been specified.

Error message

No recipients have been specified.

What it means

Validation in SmtpClient.ValidateArguments: GetMessageRecipients found no To, Cc, Bcc (or Resent-* equivalents) addresses on the message, so there is no one to send it to (RCPT TO would have no recipients) and InvalidOperationException is thrown.

Solutions

1. Add recipients before send.
2. Use explicit recipients overload.

Example fix

null

if (!(message.To.Any() || message.Cc.Any() || message.Bcc.Any()))
	throw new InvalidOperationException("missing recipients");

bool HasRecipients(MimeMessage m) => m.To?.Any() == true || m.Cc?.Any() == true || m.Bcc?.Any() == true;

catch (InvalidOperationException ex) when (ex.Message == "No recipients have been specified.") { /* dead-letter */ }

When it happens

Trigger: MimeMessage without To/Cc/Bcc.

Common situations: Empty recipient input from user data.

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

• No compatible authentication mechanisms found.
• No sender has been specified.
• null
• SmtpStatusCode.TransactionFailed
• The address cannot be empty.

AI-assisted analysis of jstedfast/MailKit@9d3859a785 (2026-09-15). Data as JSON: /api/errors/800a361f47f05ab8. Report an issue: GitHub.

Appendix: source

Thrown at MailKit/Net/Smtp/SmtpClient.cs:2526

		static void ValidateArguments (FormatOptions options, MimeMessage message, out MailboxAddress sender, out IList<MailboxAddress> recipients)
		{
			if (options == null)
				throw new ArgumentNullException (nameof (options));

			if (message == null)
				throw new ArgumentNullException (nameof (message));

			var mailbox = GetMessageSender (message);

			if (mailbox == null)
				throw new InvalidOperationException ("No sender has been specified.");

			sender = mailbox;
			recipients = GetMessageRecipients (message);

			if (recipients.Count == 0)
				throw new InvalidOperationException ("No recipients have been specified.");
		}

		/// <summary>
		/// Send the specified message.
		/// </summary>
		/// <remarks>
		/// <para>Sends the specified message.</para>
		/// <para>The sender address is determined by checking the following
		/// message headers (in order of precedence): Resent-Sender,
		/// Resent-From, Sender, and From.</para>
		/// <para>If either the Resent-Sender or Resent-From addresses are present,
		/// the recipients are collected from the Resent-To, Resent-Cc, and
		/// Resent-Bcc headers, otherwise the To, Cc, and Bcc headers are used.</para>
		/// </remarks>
		/// <example>
		/// <code language="c#" source="Examples\SmtpExamples.cs" region="SendMessageWithOptions"/>
		/// </example>
		/// <returns>The final free-form text response from the server.</returns>

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