How to fix the error: '503 STARTTLS command used when not advertised' (error number 0x800CCC65)

A very likely reason for receiving an error message like this when sending emails is your antivirus software. Avast Antivirus in particular is one of the applications known to cause this error.

The error message itself might be different depending on the email client. Outlook Express and Windows Live Mail contain the following in the error message:

Server Response:  503 STARTTLS command used when not advertised
Error Number: 0x800CCC65

And as you have probably noticed when configuring one of these email clients, there is only one checkbox for enabling encryption. It combines SSL/TLS/STARTTLS, though this is not explicitly stated. The regular encryption port number for the outgoing (SMTP) server is 465. This port number should work fine even with the antivirus application scanning your outgoing messages. STARTTLS on the other hand works on port 587. So if you have marked the checkbox for encryption and you have set the port number to 587, STARTTLS will be used, and this is when the error may occur.

The solution is to either set the port number to 465, or to disable the scanning of outgoing messages from your antivirus application. Keep in mind that some antivirus applications cause problems no matter of the port number or whether you use encryption or not. In such cases disable the scanning of email messages from the options of the antivirus/firewall application, or even consider changing the application.

The same problem also occurs with Mozilla Thunderbird. With this email client you have the option to choose between SSL/TLS and STARTTLS for encryption. If you have chosen STARTTLS (port number 587), the antivirus application again may be the reason for receiving the following error message when sending mail:

Sending of message failed. An error occurred sending mail: Unable to establish a secure link with SMTP server using STARTTLS since it doesn't advertise that feature. Switch off STARTTLS for that server or contact your service provider.

And again, either choose SSL/TLS from the encryption drop-down menu for the SMTP server and set the port number to 465, or disable the scanning of outbound messages from the options of your antivirus application.

When it comes to Microsoft Outlook, depending on the version, there may or may not be any problems with STARTTLS and the antivirus application scanning the outgoing messages. With Avast Antivirus in particular, Outlook 2002 and Outlook 2007 send messages with STARTTLS without problems. While Outlook 2003 and Outlook 2010 are affected by this issue.

Outlook 2003 (Service Pack 1) in particular has problems with STARTTLS (port 587) which are not connected with any antivirus applications, but with the email client itself. So if you use Outlook 2003 either upgrade it to a newer service pack, or simply when you enable encryption set the the port number to 465, which is the recommended encryption port number anyway.

The error message that you'll receiving in Outlook 2010 (concerning this issue) won't contain any error numbers, but will look like this:

Sending test e-mail message: Your outgoing (SMTP) e-mail server has reported an internal error. If you continue to receive this message, contact your server administrator or Internet service provider (ISP).

Unlike previous versions, in Outlook 2010, as well as in Outlook 2007, you can choose between SSL and TLS from the encryption drop-down menu when setting the mail server options. The TLS option there is actually the STARTTLS command, and hence the possible problem with the antivirus application. In any case, the solution to this issue is the same as with the other email clients: either select SSL from the encryption drop-down menu and set the port number to 465, or disable the scanning of outgoing messages from your antivirus application.

If you need help configuring a particular email client check out the respective article or tutorial.

You may also find helpful the tutorial on port numbers and encryption protocols.

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