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5.12. What MTA features are required for use with Mailman 2.1.5?

Your MTA needs to be configured to support VERP addressing. If you don't know what VERP is, see So what is this VERP stuff.

From the Mailman-2.1.5/NEWS file:

    - The bounce processor has been redesigned so that now when an address's
      bounce score reaches the threshold, that address will be sent a probe
      message.  Only if the probe bounces will the address be disabled.  The
      score is reset to zero when the probe is sent.  Also, bounce events are
      now kept in an event file instead of in memory.  This should help
      contain the bloat of the BounceRunner.

      New supporting variables in Defaults.py: VERP_PROBE_FORMAT,

      REGISTER_BOUNCES_EVERY is promoted to a Defaults.py variable.

These probe messages are sent out in VERP format, so if your MTA doesn't support VERP, you're likely to have serious problems with users being unsubscribed, etc....

Note that if you are running Mailman under CPanel, you should not upgrade to version 2.1.5, unless you got that upgrade from CPanel themselves. All Mailman support issues under CPanel should be directed to them. See Mailman and CPanel.

If your server is at a hosting provider, you need to ask your provider if the MTA is currently configured to support VERP. Do not upgrade Mailman to version 2.1.5 until you confirm that this support is in place.

There are a number of other FAQ entries which discuss VERP, and you can read them by clicking on this link: Search for VERP.

For more information on the requirements of Mailman 2.1.5, see also What version of Python is required for Mailman 2.1.5 and 2.1.9 and 2.1.10 and 2.1.12?.

The Mailman developers realize that not everyone can guarantee that their MTA provides VERP support, or can get their MTA upgraded to provide this support. Future versions of Mailman 2.1.x will most likely disable all features requiring VERP support by default, although they could easily be re-enabled by administrators wishing to do so. The Mailman developers will also try to figure out a way to automatically detect and make use of VERP support where available, while allowing Mailman to fall back to non-VERP methods where that is not possible.

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This is one of many Frequently Asked Questions.