Q: Why do subscribers sometimes get error messages about being probed and what do they mean?

By Jacob Haller
Senior Support Engineer, L-Soft

Subscribers sometimes report receiving error messages from LISTSERV with "probe failure" in the subject line. These messages usually look like this:

The email then describes why LISTSERV is sending the message and what it recommends that the affected subscriber should do. This messages should also include a copy of the most recent bounce.

What causes these messages to be sent?

If LISTSERV receives a bounce in a format that it doesn't understand, then it sometimes sends follow-up messages (or probes) to see if the address is still generating bounces.

This is to determine whether the problem that caused the original bounce is temporary or not. Normally, LISTSERV is able to figure out whether a bounce is the result of a permanent or a temporary error by looking at the error code contained in the bounce, but it can't do that if the bounce isn't in a standard format.

If the address continues to bounce, then LISTSERV will remove the address according to the email list's "Auto-Delete" setting.

Since the email address works, why would a subscriber get a "Probe Failed" message?

There are a few possibilities.

1. There could be an intermittent problem on the subscriber's end so that some messages bounce while others do not.

Looking at the logs of the mail server that handles the outgoing mail for LISTSERV, and comparing what's recorded for successful deliveries vs. unsuccessful delivery attempts, may help determine why the problem is intermittent. The copy of the bounce that's included with the "Probe Failure" message may also contain information about what caused the original message to bounce.

2. The message that LISTSERV is treating as a bounce may not be a bounce after all. If an out-of-office message is sent to the address that LISTSERV expects bounces to be sent to, then it may result in LISTSERV misinterpreting the message as a bounce in an unfamilar format. Most out-of-office autoresponders don't send mail to this address, but some may.

In this case, there isn't much that can be done about it in LISTSERV, but perhaps the affected subscriber, or their mail administrator, can adjust the software that's sending the automatic responses so that it no longer sends them to the LISTSERV bounce address.

How is LISTSERV able to identify which address is bouncing if the bounce is in a format it doesn't understand?

When LISTSERV sends out a mailing, it has a special email address, usually something like OWNER-NAMEOFLIST@LISTS.EXAMPLE.COM, to which recipient mail servers are supposed to send bounces. LISTSERV then knows that any messages received at this address, also known as the "return path", are bounces for the NAMEOFLIST mailing list.

LISTSERV also has a special version of the return path that it may use that includes an encoded version of the recipient's email address. For instance, the OWNER-NAMEOFLIST*support**lsoft*-com@LISTS.EXAMPLE.COM return path might be used for mail sent to the support@lsoft.com address if it were subscribed to the NAMEOFLIST mailing list. If LISTSERV receives a bounce at one of these special return path addresses, it will automatically know which mailing list and which subscriber it's from just by seeing what address the bounce was sent to before it even looks at the contents of the bounce.

As far as which recipients would get this special "probe-style" return path, this depends on how the mailing list is configured.

1. For mailing lists configured with "Mail-Merge= Yes", this probe-style return path will be used for all recipients of all mailings. This cannot be overridden.

2. For mailing lists that don't have mail-merge enabled, some portion of the subscribers may be probed for each mailing. See the "Probing" section of this tech tip for additional details:
http://www.lsoft.com/news/techtipLSV-issue2-2020.asp

3. Mailing lists configured with "Renewal= ...,Probe" will periodically send "active probe" messages to all subscribers that use this sort of return path. For details about this settings, see:
http://www.lsoft.com/manuals/17.0/listkwd/kRenewal.html

It is extremely rare, but you may occasionally see a site that rejects mail with a probe-style return path but accepts other mail. Generally this should be addressed by the affected subscriber's mail administrator since these types of addresses are both legal and deliverable. However, if the second scenario above applies, you can exempt individual subscribers from probing. Click on "Subscriber Management" under the "List Management" menu, enter the subscriber's email address and update their subscription settings by checking the User is Exempt from Renewal/Probing option. Note, however, that this will have no effect if the mailing list has mail-merge enabled.

Can I prevent LISTSERV from sending the "Probe Failed" messages, or change their wording?

Yes. These messages are based on the PROBE2 mail template.

To customize this template for a specific mailing list, click on "Mail Templates" under the "List Management" menu and locate the PROBE2 template. If you instead want to edit the sitewide version used by all mailing lists by default, go to "Mail Templates" under the "Server Administration" menu instead.

You can then adjust the wording of the message however you like. If you want to get rid of the messages altogether, then replace the template's contents with this single line:

If the mailing list is configured with "Mail-Merge= Yes" and every message that is distributed to the list uses probing, then suppressing PROBE2 shouldn't affect the ability of LISTSERV to find and remove bad addresses. However, for other types of mailing lists, suppressing the PROBE2 message is not recommended.

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