Yahoo initiated a terrible policy change to their mail servers that impacts every single mailing list software in use, not just Mailman. If any Yahoo.com user posts to your list, they will cause those list members whose ISP/ESP provider is using DMARC (Hotmail, Gmail, Bellsouth, etc) to bounce the poster's message which can in turn cause the list to remove these members from the list for excessive bounces. The problem is with Yahoo. You can read more about it at:
We recommend the following:
1. Resubscribing those list members that were removed.
2. Prevent any Yahoo subscriber from posting to your list until they subscribe with another non-Yahoo email address. There is no telling when Yahoo will fix this or if they ever will.
3. Worst case, you may need to turn off bounce processing for now but we do not recommend this.
Again this issue only affect those lists where Yahoo.com users are allowed to post to the list.
Thank you for using EMWD.com and we apologize for any inconvenience this has caused.
Thursday, April 10, 2014