In June 2016 gmail is going to be changing its DMARC policy to reject any unaligned mail. This means that we will be seeing any mailing list responses from a address to be considered spam by any mail server that enforces DMARC.

For a while now has had this policy turned on which has meant that addresses have been considered spam.

Mailman has released a workaround in version 2.1.18:

The boost mailing list is currently on mailman 2.1.15. I'd strongly suggest that this should be updated as a matter of urgency else we'll find a lot of the conversations on this list will end up in the spam folder...