If your list members experience unusually high levels of bounced messages, it might be due to a Yahoo policy
in conjunction with many of the major email providers called the DMARC.
It involves a rather technical explanation, but for those interested, you may read more information at the following links.
Yahoo email anti-spoofing policy breaks mailing lists - Techworld.com
Message not accepted for policy reasons when sending email to Yahoo
Note that most major providers (e.g., gmail, Comcast, Frontier, Verizon, AT&T, etc.) honor the DMARC, so this issue can effect you even if you do not have a Yahoo email address.
There are now 2 Mailman configuration options available to list owners
to address this issue. The first can be found under the General section.
Look for the option that reads:
Replace the From: header address with the list's posting address to mitigate issues stemming from the original From: domain's DMARC or similar policies.
Changing the radio button selection from "No" to "Mung From" and saving the configuration should resolve the issue of excessive bouncing due to Yahoo's recent change.
There is an alternative option similar to the one described above, but it
differs in that the header munging is performed only on addresses that
have the DMARC Rejection/Quarantine Policy enabled (e.g., yahoo.com).
Alternative to header munging, you can simply reject/discard messages
from addresses with the DMARC Rejection/Quarantine Policy enabled.
This option can be found under Privacy options->Sender filters. Look for:
Action to take when anyone posts to the list from a domain with a DMARC Reject/Quarantine Policy.
Note that if you choose to enable one of the options in this section, you should leave the corresponding option under the General section (described above) set to "No."
If you need further assistance with Mailman configuration, please contact us and let us know.